Los Angeles motorway hit hard (3)
I believe the answer is:
lam
'hit hard' is the definition.
(to 'lam' means to hit hard)
'los angeles motorway' is the wordplay.
'los angeles' becomes 'LA' (abbreviation).
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'la'+'m'='LAM'
(Other definitions for lam that I've seen before include "Escape, get away" , "Thump; leg it" , "Thrash; (US) escape" , "Go on the - - , (US sl.)" , "On this, you're fleeing, from the police" .)