Operation inevitably includes staff (11)
I believe the answer is:
performance
'operation' is the definition.
(I know that operation can be written as performance)
'inevitably includes staff' is the wordplay.
'inevitably' becomes 'perforce' (I've seen this before).
'includes' indicates putting letters inside.
'staff' becomes 'man' (as in staffing or manning a particular place).
'perforce' placed around 'man' is 'PERFORMANCE'.
(Other definitions for performance that I've seen before include "The giving of a play or concert" , "Presentation" , "Rendition" , "(Actor's) rendering of a role" , "Stage show" .)