Swell working with one in the theatre (7)
I believe the answer is:
surgeon
'one in the theatre' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'swell working' is the wordplay.
'swell' becomes 'surge' (I've seen this before).
'working' becomes 'on' (on can mean working or activated).
'surge'+'on'='SURGEON'
'with' is the link.
Can you help me to learn more?
(Other definitions for surgeon that I've seen before include "Doctor with a scalpel" , "Performer of operations" , "Naval doctor" , "Medical operator" , "Operating medical practitioner" .)