Fruit essential when heading west (7)
I believe the answer is:
satsuma
'fruit' is the definition.
(satsuma is a kind of fruit)
'essential when heading west' is the wordplay.
'essential' becomes 'a must' (it's a must).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'heading west' shows that the letters should be reversed in order.
'amust'+'as'='amustas'
'amustas' backwards is 'SATSUMA'.
(Other definitions for satsuma that I've seen before include "Citrus fruit from Japan" , "Type of tangerine" , "Type of mandarin orange" , "Orange - type of pottery" , "Loose-skinned fruit" .)