In the past, accepted fashionable weight (5)
I believe the answer is:
ounce
'weight' is the definition.
(ounce is a kind of weight)
'in the past accepted fashionable' is the wordplay.
'in the past' becomes 'once' (I've seen this before).
'accepted' means one lot of letters goes inside another.
'fashionable' becomes 'u'.
'once' placed around 'u' is 'OUNCE'.
(Other definitions for ounce that I've seen before include "one of sixteen in a pound" , "A light imperial measure of weight" , "small portion" , "One-sixteenth of pound" , "Small quantity - snow leopard" .)