Particularly old model supported by an artist (5)
I believe the answer is:
extra
'particularly' is the definition.
(synonyms)
'old model supported by an artist' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'model' becomes 'T' (Ford Model T car).
'supported by' is a charade indicator (letters next to each other) (in a down clue, some letters hold up others).
'an artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'ex'+'t'+'ra'='EXTRA'
(Other definitions for extra that I've seen before include "more than usually" , "Run scored but not off the bat" , "No great contribution to film" , "appearing in crowd scene" , "Run; in addition" .)