A spinner is on (4)
I believe the answer is:
atop
'on' is the definition.
(thesaurus)
'a spinner' is the wordplay.
'spinner' becomes 'top' (something that spins).
'a'+'top'='ATOP'
'is' is the link.
(Other definitions for atop that I've seen before include "Positioned on" , "In superior position" , "Situated on" , "Upon" , "At the summit of" .)