In theatre morphine produces shock (6)
I believe the answer is:
tremor
'shock' is the definition.
(I have seen 'Small shock ' mean 'tremor' so perhaps 'shock' could also mean 'tremor')
'in theatre morphine produces' is the wordplay.
'in' says the answer is hidden in the clue.
'TREMOR' can be found hidden inside 'theatre morphine produces'.
(Other definitions for tremor that I've seen before include "A slight shake, body or earth" , "Bit of excitement" , "Small vibration" , "Shudder - quake" , "A little agitation" .)