Doing without us is of necessity (11)
I believe the answer is:
performance
'doing' is the definition.
(synonyms)
'without us is of necessity' is the wordplay.
'without' indicates putting letters inside.
'us' becomes 'man' (mankind).
'is of necessity' becomes 'perforce' (I've seen this in another clue).
'man' placed within 'perforce' is 'PERFORMANCE'.
(Other definitions for performance that I've seen before include "Creep on farm (anag)" , "Stage show" , "operation" , "What actor gives in role" , "Particular action" .)