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' means one lot of letters goes inside another (some letters go outside others).
'close to' indicates one should 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' placed around 'n' is 'bene'.
'bene'+'fit'='BENEFIT'
(Other definitions for benefit that I've seen before include "Fringe - -" , "Avail" , "Something that aids" , "Good" , "Handout" .)