Funny Valentine, our poor half-hearted ... who? (8,7)
I believe the answer is:
relative pronoun
'who?' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I don't see?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for relative pronoun that I've seen before include "It could be that" , "Say which" , "Maybe what" .)