Be a remainer? That's pants (8)
I believe the answer is:
lingerie
'pants' is the definition.
(lingerie is a type of underwear)
'be a remainer? that's' is the wordplay.
'be a remainer?' becomes 'linger' (to linger is to remain).
'that's' becomes 'ie'.
'linger'+'ie'='LINGERIE'
(Other definitions for lingerie that I've seen before include "Women's underclothes" , "One from W Atlantic island group" , "Women's underwear and nightwear" , "Slinky undergarments" , "Ladies' underwear" .)