Fruit that's a necessity when up? (7)
I believe the answer is:
satsuma
'fruit' is the definition.
(I know that satsuma is a fruit)
'a necessity when up?' is the wordplay.
'necessity' becomes 'must' (I've seen this before).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'up?' shows that the letters should be reversed in order (in down clue: letters go upwards).
'a'+'must'+'as'='amustas'
'amustas' back-to-front is 'SATSUMA'.
'that's' is the link.
(Other definitions for satsuma that I've seen before include "As a must (anag)" , "sort of orange" , "healthy snack" , "that is easily uncovered" , "Kind of mandarin orange" .)