Lines that convey heartfelt admiration (4,6)
I believe the answer is:
love letter
'admiration' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for love letter that I've seen before include "caring note!" , "Billet-doux" , "romantic reading material" .)