Prize tree and fertiliser (6)
I believe the answer is:
potash
'fertiliser' is the definition.
(I've seen this before)
'prize tree' is the wordplay.
'prize' becomes 'pot' (I've seen this before).
'tree' becomes 'ash' (ash is a kind of tree).
'pot'+'ash'='POTASH'
'and' is the link.
(Other definitions for potash that I've seen before include "Compound used in agriculture and industry - shop at (anag)" , "Alkali used as fertiliser" , "The strange pathos of a potassium compound" , "Type of fertiliser" , "Agricultural chemical" .)