Check 1 Across say contains definition originally – firm up opening (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Check 1 Across, say contains definition originally – firm up opening OC (CO=firm, i.e. company; “up” indicates vertical reversal) + [ D <efinition> (“originally” means first letter only) in CLUE (=1 Across, say)]; to occlude is to halt, arrest, hence “check”, in Chambers Thesaurus