Horse crossing motorway in search (4)
I believe the answer is:
comb
'search' is the definition.
(I know that comb is a more specific form of the action search)
'horse crossing motorway' is the wordplay.
'horse' becomes 'cob' (type of horse).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'cob' placed around 'm' is 'COMB'.
'in' acts as a link.
(Other definitions for comb that I've seen before include "Search well; bird's crest" , "Go over" , "Device with teeth - store for honey" , "Fowl's crest" , "Arrange" .)