Stops but still in front (4)
I believe the answer is:
bars
'stops' is the definition.
'but still in front' is the wordplay.
'but' becomes 'bar' (both can mean 'excluding' or 'except').
'still in front' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'bar'+'s'='BARS'
Can you help me to learn more?
(Other definitions for bars that I've seen before include "Prohibits; sandbanks" , "Prohibits; pubs" , "Obstacles" , "Watering holes - units in musical scores" , "See 4" .)