Learnt about organ (3)
I believe the answer is:
ear
'organ' is the definition.
(the ear is an organ)
'learnt about' is the wordplay.
'about' indicates the answer is hidden within the clue (the clue goes around or about the answer).
'EAR' can be found hidden inside 'learnt'.
(Other definitions for ear that I've seen before include "Aural organ" , "Seed-bearing spike" , "Shell-like (slang)" , "organ with drum" , "Seed container" .)