Couple also heard (3)
I believe the answer is:
two
'couple' is the definition.
(a couple can mean two of something)
'also heard' is the wordplay.
'also' becomes 'too' (synonyms).
'heard' shows a homophone (sound like).
'too' is a homophone of 'TWO'.
(Other definitions for two that I've seen before include "Item" , "II" , "A low number" , "Pair, deuce" , "Minimal number of partners" .)