Fruit is a necessity when going west (7)
I believe the answer is:
satsuma
'fruit' is the definition.
(satsuma is a kind of fruit)
'a necessity when going west' is the wordplay.
'necessity' becomes 'must' (I've seen this before).
'when' becomes 'as'.
'going west' says the letters should be written in reverse.
'a'+'must'+'as'='amustas'
'amustas' written backwards gives 'SATSUMA'.
'is' is the link.
(Other definitions for satsuma that I've seen before include "Small, easily-peeled orange" , "Kind of tangerine" , "Seasonal orange" , "Citrus fruit; Jap. pottery" , "As a must (anag)" .)