Source of manure applied sparely for fast grower (3,7)
I believe the answer is:
cow parsley
'fast grower' is the definition.
'source of manure applied sparely' is the wordplay.
'source of manure' becomes 'cow'.
'applied' indicates an anagram (apply the letters).
'sparely' anagrammed gives 'parsley'.
'cow'+'parsley'='COW-PARSLEY'
'for' acts as a link.
(Other definitions for cow parsley that I've seen before include "Hedgerow plant" , "Fragrant white wayside flower" .)