Peculiar billionaire held up sign (5)
I believe the answer is:
libra
'sign' is the definition.
(sign of the zodiac)
'peculiar billionaire held up' is the wordplay.
'held' says the answer is hidden in the clue.
'up' is a reversal indicator (in down clue: letters go upwards).
'arbil' is hidden within 'peculiar billionaire'.
'arbil' back-to-front is 'LIBRA'.
(Other definitions for libra that I've seen before include "The Scales in astrology" , "Zodiac sign of the scales" , "Old pound" , "Scales constellation" , "Person born under Scales sign" .)