A certain amount is mentioned (4)
I believe the answer is:
some
'a certain' is the definition.
(eg some person means a certain person)
'amount is mentioned' is the wordplay.
'amount' becomes 'sum' (synonyms).
'is mentioned' shows a homophone (sound like).
'sum' sounds like 'SOME'.
(Other definitions for some that I've seen before include "An undetermined number" , "Unknown or unspecified amount or number" , "Part" , "Several" , "certain" .)