Fruit is an essential, since returning (7)
I believe the answer is:
satsuma
'fruit' is the definition.
(satsuma is a kind of fruit)
'an essential since returning' is the wordplay.
'an' becomes 'a'.
'essential' becomes 'must' ('must' can be similar in meaning to 'essential').
'since' becomes 'as' (I've seen this in other clues).
'returning' shows that the letters should be reversed in order.
'a'+'must'+'as'='amustas'
'amustas' backwards is 'SATSUMA'.
'is' is the link.
(Other definitions for satsuma that I've seen before include "Seedless citrus fruit" , "Citrus fruit from Japan" , "Small variety of orange similar to mandarin" , "Kind of mandarin orange" , "A small citrus fruit" .)