Prize tree getting this fertiliser (6)
I believe the answer is:
potash
'this fertiliser' is the definition.
(I've seen this before)
'prize tree getting' is the wordplay.
'prize' becomes 'pot' (I've seen this in other clues).
'tree' becomes 'ash' (ash tree is a kind of tree).
'getting' is a charade indicator (letters next to each other).
'pot'+'ash'='POTASH'
(Other definitions for potash that I've seen before include "Alkaline compound" , "Potassium compound used as fertiliser" , "Potassium compound used in agriculture and industry" , "Strong alkali derived from burnt wood" , "Agricultural chemical" .)