Assist insect outside close to sunken well (7)
I believe the answer is:
benefit
'assist' is the definition.
(thesaurus)
'insect outside close to sunken well' is the wordplay.
'insect' becomes 'bee' (bee is a type of insect).
'outside' indicates putting letters inside (some letters go outside others).
'close to' indicates one should take the final letters (the close/ending of).
'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 - -" , "Avail" , "Something that aids" , "Good" , "Handout" .)