Withstand surgery, briefly, and sit (6)
I believe the answer is:
oppose
'withstand' is the definition.
(I know that withstand is a more specific form of oppose)
'surgery briefly and sit' is the wordplay.
'surgery briefly' becomes 'op' (short for operation; 'briefly' suggests an abbreviation).
'and' says to put letters next to each other.
'sit' becomes 'pose' (as in sitting for a painter).
'op'+'pose'='OPPOSE'
(Other definitions for oppose that I've seen before include "Resist; disapprove of" , "Actively resist" , "Counter" , "Resist (someone)" , "Resist, be hostile to" .)