Three and a quarter, many heard (5)
I believe the answer is:
tried
'heard' is the definition.
(as in trying or hearing in a trial)
'three and a quarter many' is the wordplay.
'three' becomes 'tri' (prefix meaning 'three').
'and' says to put letters next to each other.
'a quarter' becomes 'e'.
'many' becomes 'D' (Roman numeral for 500).
'tri'+'e'+'d'='TRIED'
(Other definitions for tried that I've seen before include "Proven by experience" , "checked out" , "Put to the test" , "tasted" , "Essayed" .)