Black rabbit on bush (7)
I believe the answer is:
bramble
'on bush' is the definition.
'bramble' can be an answer for 'bush' (thesaurus). I am not sure about the 'on' bit.
'black rabbit' is the wordplay.
I cannot quite see how this works, but
'black' could be 'b' (as in pencil classifications) and 'b' is located in the answer.
The remaining letters 'ramble' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for bramble that I've seen before include "Source of fruit" , "Prickly plant yielding blackberries" , "Blackberry shrub" , "Prickly hazard" , "Dog rose" .)