Superior mushroom spread (5)
I believe the answer is:
swell
I believe this clue is a triple definition!
'superior' is the first definition.
'mushroom' is the second definition.
(both can mean to expand or grow in size)
'spread' is the third definition.
(thesaurus)
(Other definitions for swell that I've seen before include "Become enlarged" , "Inflate - fine" , "Heave of the sea's surface" , "Heave of sea; expand" , "part of organ" .)