Nutmeg's morning reflection on uniform flavour in food (5)
I believe the answer is:
umami
'flavour in food' is the definition.
(umami is one of the main taste categories)
'nutmeg's morning reflection on uniform' is the wordplay.
'nutmeg's' becomes 'I'm' (where the crossword setter is 'Nutmeg').
'morning' becomes 'am'.
'reflection' says the letters should be written in reverse.
'on' is a charade indicator (letters next to each other).
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'im'+'am'='imam'
'imam' backwards is 'mami'.
'mami' put after 'u' is 'UMAMI'.
(Other definitions for umami that I've seen before include "Savoury taste sensation produced by glutamates - I, a mum (anag)" , "meaty taste" , "Fifth flavour" , "Newly discovered taste" , "acquired taste perhaps" .)