A pigeon a supporter has to follow (7)
I believe the answer is:
'a pigeon' is the definition.
(type of pigeon)
'a supporter has to follow' is the wordplay.
'a supporter' becomes 'fan' (fan is a kind of supporter).
'has' is a charade indicator (letters next to each other).
'to follow' becomes 'tail' (both can mean to chase or pursue).
(Other definitions for fantail that I've seen before include "Breed of pigeon" , "Bird" , "Flier" , "In a flat (anag.)" , "Variety of pigeon" .)