Brilliant wool supplier fills empty jail (5)
I believe the answer is:
jewel
'brilliant' is the definition.
(thesaurus)
'wool supplier fills empty jail' is the wordplay.
'wool supplier' becomes 'ewe' (I've seen this before).
'fills' indicates putting letters inside.
'empty' suggests removing the centre.
'jail' with its middle removed is 'jl'.
'ewe' inserted within 'jl' is 'JEWEL'.
(Other definitions for jewel that I've seen before include "Precious stone or mineral" , "Gem" , "(Semi) precious stone" , "Gemstone" , "eg Ruby" .)