Fruit is a necessity when heading north (7)
I believe the answer is:
satsuma
'fruit' is the definition.
(satsuma is a kind of fruit)
'a necessity when heading north' is the wordplay.
'necessity' becomes 'must' (I've seen this before).
'when' becomes 'as'.
'heading north' says the letters should be written backwards.
'a'+'must'+'as'='amustas'
'amustas' in reverse letter order is 'SATSUMA'.
'is' acts as a link.
(Other definitions for satsuma that I've seen before include "Citrus tree named after an old Japanese province" , "Kind of mandarin orange" , "that is easily uncovered" , "Type of mandarin orange" , "A small citrus fruit" .)