Wheel radius to change gradually (7)
I believe the answer is:
revolve
'wheel' is the definition.
(thesaurus)
'radius to change gradually' is the wordplay.
'radius' becomes 'r' (abbreviation in geometry).
'to change gradually' becomes 'evolve' (I've seen this before).
'r'+'evolve'='REVOLVE'
(Other definitions for revolve that I've seen before include "Go round" , "Rotate" , "Pirouette" , "Turn on an axis" , "stage feature" .)