On reflection, a must as a fruit (7)
I believe the answer is:
satsuma
'fruit' is the definition.
(satsuma is a kind of fruit)
'on reflection a must as a' is the wordplay.
'on reflection' says the letters should be written in reverse.
'must'+'as'='mustas'
'mustas' in reverse letter order is 'satsum'.
'satsum'+'a'='SATSUMA'
(Other definitions for satsuma that I've seen before include "Loose-skinned tangerine" , "Japanese province and small orange" , "Loose-skinned fruit" , "Seasonal orange" , "sort of orange" .)