First pair of thespians engaged in leading role, mainly (3,3,4,4)
I believe the answer is:
for the most part
'mainly' is the definition.
(I've seen this before)
'first pair of thespians engaged in leading role' is the wordplay.
I cannot quite understand how this works, but
'first' could be 'a' and 'a' is found within the answer.
'pair' could be 'pr' and 'pr' is present in the leftover letters.
'of' could be 'o' and 'o' is present in the answer.
This explanation may well be incorrect...
(Other definitions for for the most part that I've seen before include "in general" , "Usually" , "Mainly" , "Chiefly" .)