Left one item of underwear as a sign (5)
I believe the answer is:
libra
'a sign' is the definition.
(sign of the zodiac)
'left one item of underwear' is the wordplay.
'left' becomes 'l' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'item of underwear' becomes 'bra' (bra is a type of underwear item).
'l'+'i'+'bra'='LIBRA'
'as' acts as a link.
(Other definitions for libra that I've seen before include "The Scales in astrology" , "Scales star sign" , "Zodiac sign of the scales" , "Seventh sign of the Zodiac" , "Constellation near Virgo" .)