Second actor is a fake (4)
I believe the answer is:
sham
'fake' is the definition.
(sham can mean fake or pretend)
'second actor' is the wordplay.
'second' becomes 's' (common abbreviation).
'actor' becomes 'ham' (ham is a kind of actor).
's'+'ham'='SHAM'
'is a' acts as a link.
(Other definitions for sham that I've seen before include "'Fake, pseudo (4)'" , "Not what it seems to be" , "Phoney sort of hams" , "False and phoney" , "Mock" .)