Curbs ways to limit speed (9)
I believe the answer is:
moderates
'curbs' is the definition.
('moderate' can be a synonym of 'curb')
'ways to limit speed' is the wordplay.
'ways' becomes 'modes' ('mode' can be a synonym of 'way').
'to limit' is an insertion indicator (inserted letters are restrained or limited).
'speed' becomes 'rate' (the speed at which something happens).
'modes' going around 'rate' is 'MODERATES'.
(Other definitions for moderates that I've seen before include "chairs" , "People who don't have extreme views, say about politics" .)