Fertiliser in firm mass on display (7)
I believe the answer is:
compost
'fertiliser' is the definition.
(type of fertiliser)
'firm mass on display' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'on' says to put letters next to each other.
'display' becomes 'post' (I've seen this in other clues).
'co'+'m'+'post'='COMPOST'
'in' is the link.
(Other definitions for compost that I've seen before include "Decaying vegetation as fertilizer" , "Growing medium" , "material that promotes growth" , "Fertiliser (from heap or bin)" , "Mixture of decaying matter added to soil" .)