A traffic stopper in the road around central Harlesden (3)
I believe the answer is:
red
'a traffic stopper' is the definition.
(I've seen this in another clue)
'road around central harlesden' is the wordplay.
'road' becomes 'rd' (abbreviation for road).
'around' means one lot of letters goes inside another.
'central' means to look at the middle letters.
The middle letter of 'harlesden' is 'e'.
'rd' placed around 'e' is 'RED'.
'in the' acts as a link.
(Other definitions for red that I've seen before include "Scarlet or vermilion" , "Left-wing" , "Warning light colour" , "Very angry, seeing this" , "Colour for danger" .)