Learnt to play the horn (6)
I believe the answer is:
antler
'the horn?' is the definition.
(antler is a kind of horn)
'learnt to play' is the wordplay.
'to play' indicates anagramming the letters.
'learnt' anagrammed gives 'ANTLER'.
(Other definitions for antler that I've seen before include "Deer-horn" , "Stag horn" , "Branch" , "Deer's branching horn" , "Animal's extension" .)