Garment bishop covered in fish eggs (4)
I believe the answer is:
robe
'garment' is the definition.
(robe is a type of garment)
'bishop covered in fish eggs' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'covered in' means one lot of letters goes inside another.
'fish eggs' becomes 'roe' (synonyms).
'b' inserted within 'roe' is 'ROBE'.
(Other definitions for robe that I've seen before include "Formal gown" , "Clobber" , "Formal attire" , "Long loose garment or dressing gown" , "Loose or formal garment" .)