Stroll, having capacity to cross motorway (5)
I believe the answer is:
amble
'stroll' is the definition.
(synonyms)
'having capacity to cross motorway' is the wordplay.
'having capacity' becomes 'able' (being capable).
'to cross' indicates putting letters inside (some letters cross or go round others).
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'able' placed around 'm' is 'AMBLE'.
(Other definitions for amble that I've seen before include "Move at an easy pace" , "promenade" , "Blame for leisurely walk" , "Stroll, saunter" , "Take leisurely stroll" .)