Sign of artist supporting party member (5)
I believe the answer is:
libra
'sign' is the definition.
(sign of the zodiac)
'artist supporting party member' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'party member' becomes 'lib' (short for Liberal).
'ra' after 'lib' is 'LIBRA'.
'of' is the link.
(Other definitions for libra that I've seen before include "Old pound" , "The Scales in astrology" , "Constellation near Virgo" , "Person born under Scales sign" , "Zodiacal sign" .)