Fruit may be an obligation when up (7)
I believe the answer is:
satsuma
'fruit' is the definition.
(I know that satsuma is a type of mandarin orange)
'an obligation when up' is the wordplay.
'an' becomes 'a'.
'obligation' becomes 'must' (something someone is obliged to do).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'up' is a reversal indicator (in down clue: letters go upwards).
'a'+'must'+'as'='amustas'
'amustas' back-to-front is 'SATSUMA'.
'may be' acts as a link.
(Other definitions for satsuma that I've seen before include "Seedless citrus fruit" , "Easy-peeling tangerine" , "Citrus tree named after an old Japanese province" , "Kind of mandarin orange" , "Small variety of orange similar to mandarin" .)