Undertaking surgery (9)
I believe the answer is:
operation
I believe this clue is a double definition.
'undertaking' is the first definition.
(I've seen this before)
'surgery' is the second definition.
('operation' can be a synonym of 'surgery')
(Other definitions for operation that I've seen before include "Action; process" , "Theatre procedure" , "To one pair (anag.)" , "In hospital procedure" , "battle" .)