Niche working in retirement's fine (4)
I believe the answer is:
nook
'niche' is the definition.
(I've seen this in another clue)
'working in retirement's fine' is the wordplay.
'working in' becomes 'on' (I've seen this in other clues).
'retirement's' says the letters should be written in reverse (I've seen 'retirement' mean this (retire can mean to return or go back)).
'fine' becomes 'OK' (synonyms).
'on' reversed gives 'no'.
'no'+'ok'='NOOK'
(Other definitions for nook that I've seen before include "Secluded spot" , "Sheltered corner" , "Recess or cranny" , "Quiet corner" , "cosy retreat" .)