Upsetting part of the mask costume and foot warmers (5)
I believe the answer is:
socks
'foot warmers' is the definition.
(I've seen this before)
'upsetting part of the mask costume' is the wordplay.
'upsetting' says the letters should be written in reverse.
'part of' says the answer is hidden in the clue.
'skcos' is hidden in the letters of 'the mask costume'.
'skcos' reversed gives 'SOCKS'.
'and' is the link.
(Other definitions for socks that I've seen before include "Hits; some clothing" , "White horse's leg marks" , "Short stockings" , "pair of garments" , "Short hosiery" .)