Could be posh sounding lot of fertiliser (9)
I believe the answer is:
phosphate
'lot of fertiliser' is the definition.
The definition suggests a singular noun which matches the answer.
'could be posh sounding' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'posh' is 'phos' which is present in the answer.
'sounding' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
The remaining letters 'phat' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for phosphate that I've seen before include "Salt used in fertilizers" , "Hope paths lead to artificial fertiliser" , "Acid salt used, eg, as a fertiliser" .)