Across
4a
Art to be found in certain puzzles — and in the other across solutions (6,8)
See 1
Inclined to show stress in hospital, I confess (6)
Funny guy's agreement secured by another (8)
Things to take on board or move into road transport (6)
It's rocky as result of drop in share (8)
Total including century after start of play, following plan (10)
I haven't cracked this one yet — but Fifteensquared have:
TALLY (total) round C (century) after ACT I (start of play) – Edit: the film is CAL – thanks to commenters @2 and 3
Jazz pianist's arrangement of Waterfalls (4,6)
Wrote as American essayist, having moved gold to front, followed by diamonds (8)
They help with proof in problem mastery (6)
Outstanding success in strike after strike (5,3)
Cobblers can make a lot of money covering pair of feet (6)
Entire EP remixed — it's an evergreen (4,4)
Page in Welsh converted for young English setters, say (6)
Down
Clips of omissions from a hippie classic? (8)
Who, for example, is ahead of a film star princess in frantic contest? (4,4)
One nasty guy cutting blockbuster in pieces (8)
I haven't cracked this one yet — but Fifteensquared have:
I SOD (one nasty guy) in EPIC (blockbuster)
I study old cinema, initially with ultimate critic (10)
I haven't cracked this one yet — but Fifteensquared have:
I CON (I study) + O (old) + C[inema] + LAST (ultimate)
Dress protecting male and female in horror film (3,3)
I haven't cracked this one yet — but Fifteensquared have:
TOG (dress) round HE (male) + F (female)
Give a detailed account of story in trailer that's abbreviated and modified (6)
I haven't cracked this one yet — but Fifteensquared have:
An anagram (modified) of TRAILE[r] minus its last letter (abbreviated)
Child was portrayed by this actress when about 60 (6)
I haven't cracked this one yet — but Fifteensquared have:
After a bit of digging, I discovered that, in 2009, Meryl Streep portrayed Julia Child in ‘Julie & Julia’ and that she was born in 1949, so ‘about 60’
Vehicle followed by second in French comedy, treat with superficial sweetness (10)
I haven't cracked this one yet — but Fifteensquared have:
CAR (vehicle) + S (second) in AMÉLIE (French comedy)
Giddy fame secured by star from the silent era that one tries to land? (4,4)
One's removed from a blue film, oddly culpable (8)
I haven't cracked this one yet — but Fifteensquared have:
An anagram (oddly) of A BLUE F[i]LM
Nonsense about English director cutting one line in scene of The Singing Nun (8)
Musical animated movie — saucy stuff (6)
Sound of music coming from orchestra inside (6)
Screwball comedy that needs work put into it (3,3)
Type letters or click words in the clue