Compound used for grass when harvesting starts (6)
I believe the answer is:
potash
'compound' is the definition.
(potash is a kind of compound)
'grass when harvesting starts' is the wordplay.
'grass' becomes 'pot' (cannabis).
'when' becomes 'as'.
'starts' suggests taking the first letters.
The first letter of 'harvesting' is 'h'.
'pot'+'as'+'h'='POTASH'
'used for' acts as a link.
(Other definitions for potash that I've seen before include "Alkali used as fertiliser" , "Strong alkali derived from burnt wood" , "feed in bed?" , "Caustic stuff" , "field dressing" .)