Fertiliser's price restricting old representative (7)
I believe the answer is:
compost
'fertiliser's' is the definition.
(type of fertiliser)
'price restricting old representative' is the wordplay.
'price' becomes 'cost' (synonyms).
'restricting' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'representative' becomes 'mp' (Member of Parliament, a representative).
'o'+'mp'='omp'
'cost' placed around 'omp' is 'COMPOST'.
(Other definitions for compost that I've seen before include "material that promotes growth" , "Decomposed vegetable matter used as fertilizer" , "Decayed organic fertilising material" , "Decaying vegetation as fertilizer" , "Mixture of decaying matter added to soil" .)