Supporter favouring Prince (4)
I believe the answer is:
prop
'supporter' is the definition.
(thesaurus)
'favouring prince' is the wordplay.
'favouring' becomes 'pro' (I've seen this in other clues).
'prince' becomes 'p'.
'pro'+'p'='PROP'
(Other definitions for prop that I've seen before include "Seize and hold firmly" , "Support (a rugby player?)" , "A rigid supprot or piece of satge furniture (4)" , "Supporter" , "See 5" .)