Expert wearing a new pinny (5)
I believe the answer is:
apron
'pinny' is the definition.
(I've seen this before)
'expert wearing a new' is the wordplay.
'expert' becomes 'pro' (short for professional).
'wearing' means one lot of letters goes inside another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'a'+'n'='an'
'pro' put within 'an' is 'APRON'.
(Other definitions for apron that I've seen before include "Protective garment - part of stage" , "conveyor-belt" , "Pinnie" , "Cook's garment" , "Protective garment - type of stage" .)