Theatre specialist, participant in yen, euro, surge once (12)
I believe the answer is:
neurosurgeon
'theatre specialist' is the definition.
(someone working in an operating theatre)
'participant in yen euro surge once' is the wordplay.
'participant in' indicates a hidden word (I've seen 'participants in' mean this).
'NEUROSURGEON' is hidden in the letters of 'yen euro surge once'.
(Other definitions for neurosurgeon that I've seen before include "top person in theatre" , "One operating on brain, spinal cord, etc" , "doctor" .)