Fruit, it's a necessity when retired (7)
I believe the answer is:
satsuma
'fruit' is the definition.
(satsuma is a kind of fruit)
'a necessity when retired' is the wordplay.
'necessity' becomes 'must' (I've seen this in another clue).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'retired' is a reversal indicator.
'a'+'must'+'as'='amustas'
'amustas' written backwards gives 'SATSUMA'.
'it's' acts as a link.
(Other definitions for satsuma that I've seen before include "Seasonal orange" , "Citrus fruit; Jap. pottery" , "Kind of mandarin orange" , "Orange, district and style of Japanese pottery" , "Citrus tree named after an old Japanese province" .)