An expert in protective garment (5)
I believe the answer is:
apron
'protective garment' is the definition.
(apron is a kind of protective garment)
'an expert in' is the wordplay.
'expert' becomes 'pro' (short for professional).
'in' means one lot of letters goes inside another.
'an' enclosing 'pro' is 'APRON'.
(Other definitions for apron that I've seen before include "Pinafore" , "Type of stage; pinny" , "Kitchen protector" , "Aircraft parking area" , "Protective garment - part of stage" .)