Bishop and cardinal spread fertiliser (8)
I believe the answer is:
bonemeal
'fertiliser' is the definition.
(I have seen 'Organic fertiliser' mean 'bonemeal' so perhaps 'fertiliser' could also mean 'bonemeal')
'bishop and cardinal spread' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'and' is a charade indicator (letters next to each other).
'cardinal' becomes 'one' (a cardinal number).
'spread' becomes 'meal' (a spread of food).
'b'+'one'+'meal'='BONEMEAL'
(Other definitions for bonemeal that I've seen before include "Type of fertiliser" , "Organic fertiliser" .)