Fine mushroom spread (5)
I believe the answer is:
swell
This clue is a triple definition!
'fine' is the first definition.
(I've seen this before)
'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 "Puff up" , "Become larger in size" , "Enlarge; sea movement" , "Dilate - jolly good!" , "Informally good, grow larger" .)