Learner with brilliant material (4)
I believe the answer is:
lace
'material' is the definition.
(lace is a kind of material)
'learner with brilliant' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'with' is a charade indicator (letters next to each other).
'brilliant' becomes 'ace' (I've seen this before).
'l'+'ace'='LACE'
(Other definitions for lace that I've seen before include "Fine needlework" , "Shoestring: delicate ornamental fabric" , "Shoe string" , "Tear" , "Shoe tie" .)