Swell guy in theatre not performing (5)
I believe the answer is:
surge
'swell' is the definition.
(I've seen this before)
'guy in theatre not performing' is the wordplay.
'guy in theatre' becomes 'surgeon' (person working in an operating theatre).
'not' is a deletion indicator.
'performing' becomes 'on' (I've seen this in other clues).
'surgeon' with 'on' taken away is 'SURGE'.
(Other definitions for surge that I've seen before include "Sudden increase in power" , "Sudden rush" , "Sudden flow" , "Sudden influx" , "Suddenly increase" .)