Assist insect swallowing close to sunken well (7)
I believe the answer is:
benefit
'assist' is the definition.
(thesaurus)
'insect swallowing close to sunken well' is the wordplay.
'insect' becomes 'bee' (bee is a type of insect).
'swallowing' means one lot of letters goes inside another.
'close to' says to take the final letters (the close of something is the end).
'well' becomes 'fit' (associated in meaning).
The final letter of 'sunken' is 'n'.
'bee' enclosing 'n' is 'bene'.
'bene'+'fit'='BENEFIT'
(Other definitions for benefit that I've seen before include "Fringe - -" , "Payment" , "Something that aids" , "Charity event" , "Good" .)