Learner donning Christopher's traditional garment (4)
I believe the answer is:
kilt
'traditional garment' is the definition.
(I know that kilt is a traditional garment)
'learner donning christopher's' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'donning' means one lot of letters goes inside another (I've seen this in other clues).
'christopher' becomes 'Kit' (short for Christopher).
'l' put inside 'kit' is 'KILT'.
(Other definitions for kilt that I've seen before include "Highland skirt" , "Under which, traditionally, a Scotsman wears nothing" , "Scots garment" , "Short skirt worn in Scotland (and by old Egyptian warriors)" , "Scots dress" .)