Mock poor actor after onset of stress (4)
I believe the answer is:
sham
'mock' is the definition.
(both can mean fake)
'poor actor after onset of stress' is the wordplay.
'poor actor' becomes 'ham' (ham can mean a bad or over-the-top actor).
'after' means one lot of letters go next to another.
'onset of' indicates taking the first letters.
The initial letter of 'stress' is 's'.
'ham' after 's' is 'SHAM'.
(Other definitions for sham that I've seen before include "imitation" , "Pretence, hoax" , "Pretence; bogus, false" , "Pretense; bogus" , "Cheap falseness" .)