Fight excess swallowing small single (6)
I believe the answer is:
resist
'fight' is the definition.
(resisting is a kind of fighting)
'excess swallowing small single' is the wordplay.
'excess' becomes 'rest' ().
'swallowing' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'single' becomes 'i' (one).
's'+'i'='si'
'rest' enclosing 'si' is 'RESIST'.
(Other definitions for resist that I've seen before include "Take stand against, oppose" , "Put up opposition to sister" , "Withstand, struggle against" , "Forgo" , "Withstand, oppose" .)