Anything you say should (5)
I believe the answer is:
aught
'anything' is the definition.
(I've seen this before)
'you say should' is the wordplay.
'you say' indicates a 'sounds like' (homophone) clue.
'should' becomes 'ought' (synonyms).
'ought' is a homophone of 'AUGHT'.
(Other definitions for aught that I've seen before include "Anything at all (archaic)" , "Anything (Shakespearean)" , "Emphatically anything" , "Nothing" .)