Accepted supporter thanks a princess (7)
I believe the answer is:
infanta
'a princess' is the definition.
(I've seen this before)
'accepted supporter thanks' is the wordplay.
'accepted' becomes 'in' (I've seen this in other clues).
'supporter' becomes 'fan' (fan is a kind of supporter).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'in'+'fan'+'ta'='INFANTA'
(Other definitions for infanta that I've seen before include "Formerly, the daughter of a Spanish king" , "Royal personage" , "Daughter of a Spanish or Portuguese king" , "member of the Spanish royal family" , "Historically, a daughter of the Spanish king - sounds very young" .)