You haven't only one, dear heart (3)
I believe the answer is:
ear
'heart' is the definition.
The definition and answer can be both body parts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'you haven't only one dear' is the wordplay.
'EAR' is hidden within the letters.
However, I'm uncertain how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for ear that I've seen before include "A corny bit" , "Appreciation of sounds" , "Shell-like (slang)" , "Seed-bearing spike" , "Speculator ignoring British" .)