Across
Doctor John was his assistant for many cases (8)
I haven't cracked this one yet — but Fifteensquared have:
whole clue as definition: Dr John Watson [wiki] worked with Sherlock Holmes in many cases
Serious fear leader in Times has something wrong with it (6)
I haven't cracked this one yet — but Fifteensquared have:
leading letter in T-[imes] with ERROR=”something wrong”
Going back, identify old man as one of our ancestors (6)
I haven't cracked this one yet — but Fifteensquared have:
reversal/”Going back” of: NAME=”identify” + PA=dad=”old man”
Getting behind wheel with added energy (8)
I haven't cracked this one yet — but Fifteensquared have:
DRIVING=”behind wheel” with E (energy) added inside
Overturn function of multinational organization (4)
Warrant for admission is missing from home inspection (10)
I haven't cracked this one yet — but Fifteensquared have:
IS (taken directly from the surface) missing from IN=”home” + V-[ is ]-ITATION=”inspection”
Garden shrubs – or try elms for a change (7)
I haven't cracked this one yet — but Fifteensquared have:
anagram/”change” of (try elms)*
Do some artwork – in the blue, lacking detail (7)
Testing periods as result of strange absorption (10)
Start of spectrum initially rearranged for rave-up (4)
I haven't cracked this one yet — but Fifteensquared have:
anagram of (ROYG)* “Start of spectrum initially” gives R-[ed] O-[range] Y-[ellow] G-[reen], the initials of the starting four ‘colours of the rainbow’
Unable to face house or pad guest trashed (8)
I haven't cracked this one yet — but Fifteensquared have:
definition: to upstage an actor is to make them have to turn away from the audience / the “house” anagram/”trashed” of (pad guest)*
Be tediously persistent as battleaxe (6)
Artist in relation to one of his colours (6)
Mistakenly download – it might contain old chestnuts (8)
I haven't cracked this one yet — but Fifteensquared have:
anagram/”Mistakenly” of (download)*
Down
Epiphany came as our smallest denomination financially declined (3,5,7)
Totally how to enter answer for this clue and the one before (4,3,2,6)
I'd connect somehow with another one falling at the same time (10)
I haven't cracked this one yet — but Fifteensquared have:
anagram/”somehow” of (I’d connect I )*, with the second I =”[another] one”
Indulge in free fall as part of risky diversion (7)
Performer at the top or scoundrels rising to it (4)
How one's brought back to earth in field (15)
I haven't cracked this one yet — but Fifteensquared have:
whole clue as definition: reference to the Earth’s gravitational field
Completely prone to face this way? (4,2,3,6)
I haven't cracked this one yet — but Fifteensquared have:
whole clue as definition: “prone” can mean lying on the ground facing downward
Has a wee dram, replaces ball on course (5,1,4)
I haven't cracked this one yet — but Fifteensquared have:
definition: to take a drop in golf means to move the ball back onto the course after it goes into a penalty area out of bounds – correction thanks to AlanC in the comments to take a drop might also mean to have a drop/”wee dram” of a drink
Inside story appropriately placed, doubly so (7)
Outfit starts off game at rock bottom (4)
Type letters or click words in the clue