Slow down Liverpool fan crossing pitch (6)
I believe the answer is:
retard
'slow down' is the definition.
(retarding is a kind of slowing down)
'liverpool fan crossing pitch' is the wordplay.
'liverpool fan' becomes 'red' (fan of Liverpool FC).
'crossing' indicates putting letters inside.
'pitch' becomes 'tar' (synonyms).
'red' placed around 'tar' is 'RETARD'.
(Other definitions for retard that I've seen before include "Darted about to hold back or delay" , "Slow down for trader" , "Hinder progress" , "Slow down growth" , "Delay effect of" .)