Mock debut of Stallone, a terrible actor (4)
I believe the answer is:
sham
'mock' is the definition.
(both can mean fake)
'debut of stallone a terrible actor' is the wordplay.
'debut of' indicates taking the first letters (I've seen 'debut' mean this).
'a terrible actor' becomes 'ham' (ham can mean a bad actor).
The first letter of 'stallone' is 's'.
's'+'ham'='SHAM'
(Other definitions for sham that I've seen before include "Unreal" , "Bogus hams" , "Pretence; bogus, false" , "Mock" , "Feign" .)