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 an anagram.
'learnt' with letters rearranged gives 'ANTLER'.
(Other definitions for antler that I've seen before include "Stag horn" , "Deer-horn" , "Branched horn of deer" , "Branch" , "Reindeer horn" .)