Popular supporter cheers Spanish princess (7)
I believe the answer is:
infanta
'spanish princess' is the definition.
(title for a Spanish princess)
'popular supporter cheers' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'supporter' becomes 'fan' (fan is a kind of supporter).
'cheers' becomes 'ta' (both can informally mean thank you).
'in'+'fan'+'ta'='INFANTA'
(Other definitions for infanta that I've seen before include "King of Spain's daughter" , "Daughter of a Spanish or Portuguese king" , "Historically, a daughter of the Spanish king - sounds very young" , "Royal personage" , "Formerly, the daughter of a Spanish king" .)