Black on top, not soft, dessert shows potential (7)
I believe the answer is:
budding
'shows potential' is the definition.
'black on top not soft dessert' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'on top' is a charade indicator (letters next to each other).
'not' indicates named letters should be taken away.
'soft' becomes 'p' (musical abbreviation for 'piano').
'dessert' becomes 'pudding' (pudding is a kind of dessert).
'pudding' with 'p' removed is 'udding'.
'b'+'udding'='BUDDING'
(Other definitions for budding that I've seen before include "Potential" , "Showing signs of talent" .)