Learnt to play a horn (6)
I believe the answer is:
antler
'a horn' is the definition.
(I know that antler is a type 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 "Stag horn" , "Deer-horn" , "Branched horn of deer" , "Branch" , "Reindeer horn" .)