On reflection, somewhat grim, a musty taste sensation (5)
I believe the answer is:
umami
'taste sensation' is the definition.
(one of the five main tastes)
'on reflection somewhat grim a musty' is the wordplay.
'on reflection' says the letters should be written in reverse.
'somewhat' says the answer is hidden in the clue.
'imamu' is hidden in the letters of 'grim a musty'.
'imamu' backwards is 'UMAMI'.
(Other definitions for umami that I've seen before include "Savoury flavour" , "meaty taste" , "Japanese word for pleasant savoury taste" , "Savoury taste sensation produced by glutamates - I, a mum (anag)" , "Fifth flavour" .)