Reduce speed, after a fashion (8)
I believe the answer is:
moderate
'reduce' is the definition.
(I've seen this before)
'speed after a fashion' is the wordplay.
'speed' becomes 'rate' (both can mean pace).
'after' says to put letters next to each other.
'a fashion' becomes 'mode' ('mode' can be a synonym of 'fashion').
'rate' put after 'mode' is 'MODERATE'.
(Other definitions for moderate that I've seen before include "Average in amount" , "So-so; sensible" , "Not excessively right- or left-wing" , "Average; not radical" , "Chair" .)