A mousse on black bread (5)
I believe the answer is:
bagel
'bread' is the definition.
(bagel is a kind of bread)
'a mousse on black' is the wordplay.
'mousse' becomes 'gel' (synonyms).
'on' says to put letters next to each other.
'black' becomes 'b' (as in pencil classifications).
'a'+'gel'='agel'
'agel' put after 'b' is 'BAGEL'.
(Other definitions for bagel that I've seen before include "Hard doughnut-shaped roll" , "round fare" , "Jewish baked roll" , "Ring-shaped bread product" , "Hard bread roll in shape of ring" .)