Laugh in pet shop, spreading salt (9)
I believe the answer is:
phosphate
'salt' is the definition.
(I know that phosphate is a type of salt)
'laugh in pet shop spreading' is the wordplay.
'laugh' becomes 'ha'.
'in' is an insertion indicator.
'spreading' indicates anagramming the letters.
'pet'+'shop'='petshop'
'petshop' is an anagram of 'phospte'.
'ha' inserted inside 'phospte' is 'PHOSPHATE'.
(Other definitions for phosphate that I've seen before include "Acid salt used, eg, as a fertiliser" , "Hope paths lead to artificial fertiliser" , "Salt used in fertilizers" .)