Reason, so we hear, for hearts (5)
I believe the answer is:
cores
'for hearts' is the definition.
(core can mean the heart of something)
'reason so we hear' is the wordplay.
'reason' becomes 'cause' (I've seen this in another clue).
'so we hear' shows a homophone (sound like).
'cause' is a homophone of 'CORES'.
(Other definitions for cores that I've seen before include "Central parts" , "Hearts" , "The centres of objects" .)