Heard song before in poem (3)
I believe the answer is:
ere
'before in poem' is the definition.
(archaic or poetic term for 'before')
'heard song' is the wordplay.
'heard' shows a homophone (sound like).
'song' becomes 'air' (I've seen this before).
'air' sounds like 'ERE'.
(Other definitions for ere that I've seen before include "Before (lit.)" , "Before, poetically" , "Before (in time)" , "One who steals" , "Old word for before" .)