Dream team, say, regularly backing supporter (7)
I believe the answer is:
fantasy
'dream' is the definition.
(thesaurus)
'team say regularly backing supporter' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'backing' says to put letters next to each other.
'supporter' becomes 'fan' (fan is a kind of supporter).
'team'+'say'='teamsay'
The alternating letters of 'teamsay' are 'tasy'.
'tasy' after 'fan' is 'FANTASY'.
(Other definitions for fantasy that I've seen before include "Vision" , "Fay tans badly and forms a mental image" , "Daydream, illusion or fancy" , "Unreal idea" , "Imagination unrestricted by reality" .)