Certainly didn't return for a bit of revenge (5)
I believe the answer is:
never
'certainly' is the definition.
Both the answer and definition are adverbs. Maybe you can see an association between them that I can't see?
'didn't return for a bit of revenge' is the wordplay.
'didn't return' becomes 'neve' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'for' says to put letters next to each other.
'a bit of' says to take the initial letters.
The first letter of 'revenge' is 'r'.
'neve'+'r'='NEVER'
Can you help me to learn more?
(Other definitions for never that I've seen before include "At no time whatever" , "No way" , "At no time in the past or future" , "Not in a million years!" , "Emphatic negative" .)