Number working in a theatre (12)
I believe the answer is:
anaesthetist
'number' is the definition.
(someone who numbs people before surgery)
'working in a theatre' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for anaesthetist that I've seen before include "Hospital theatre worker" , "One preparing patients for operations" , "Number" .)