Learnt about the horn (6)
I believe the answer is:
antler
'the horn' is the definition.
(antler is a kind of horn)
'learnt about' is the wordplay.
'about' indicates anagramming the letters.
'learnt' anagrammed gives 'ANTLER'.
(Other definitions for antler that I've seen before include "Deer's branching horn" , "Branched animal horn" , "Bony outgrowth in the deer family" , "Reindeer horn" , "Part of a deer" .)