Force over road crossing (4)
I believe the answer is:
ford
'crossing' is the definition.
(ford is a kind of crossing)
'force over road' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'over' becomes 'o' (cricket abbreviation).
'road' becomes 'rd' (abbreviation).
'f'+'o'+'rd'='FORD'
(Other definitions for ford that I've seen before include "Car - river crossing" , "Famous American of yesteryear" , "Ex-president" , "Shallow place for river crossing" , "Shallow area in a stream where it can be crossed" .)