It's a heap of use to gardeners (7)
I believe the answer is:
compost
'a heap of use to gardeners' is the definition.
I can't tell whether this defines the answer.
'it's' is the wordplay.
I cannot quite see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
The remaining letters 'compos' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for compost that I've seen before include "Decaying vegetation as fertilizer" , "Fertilizer for plants" , "Decayed organic matter" , "Fertiliser (from heap or bin)" , "material that promotes growth" .)