Magic mushroom (5)
I believe the answer is:
I believe this is a double definition.
'magic' is the first definition.
(both can mean excellent)
'mushroom' is the second definition.
(both can mean to expand or grow in size)
(Other definitions for swell that I've seen before include "Enlarge; sea movement" , "Gentle sea movement" , "organ effect" , "Heave of the sea's surface" , "Movement of the surface of the sea" .)