Responses concerning suits (9)
I believe the answer is:
'responses' is the definition.
(I know this)
'concerning suits' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'suits' becomes 'actions' ('action' can be a synonym of 'suit').
(Other definitions for reactions that I've seen before include "Responses, chemical perhaps" , "Mental or physical responses" , "Takes" , "Newton's law of motion says all actions have them" , "What you may get with chemicals" .)