Tailor buttoning up fashionable, retro garment (5)
I believe the answer is:
tunic
'garment' is the definition.
(historical type of garment)
'tailor buttoning up fashionable retro' is the wordplay.
'tailor' becomes 'cut' (synonyms).
'buttoning up' means one lot of letters goes inside another (inserted letters are sealed or buttoned in place).
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'retro' shows that the letters should be reversed in order (retro is Latin for 'backwards').
'cut' going around 'in' is 'cinut'.
'cinut' backwards is 'TUNIC'.
(Other definitions for tunic that I've seen before include "Short uniform coat" , "Cut in (anag)" , "Partly uniform" , "Military uniform" , "Smock" .)