Drunk crosses motorway verge (5)
I believe the answer is:
limit
'verge' is the definition.
(I've seen this before)
'drunk crosses motorway' is the wordplay.
'drunk' becomes 'lit' (slang term).
'crosses' is an insertion indicator.
'motorway' becomes 'MI' (resembles Ml, English motorway).
'lit' enclosing 'mi' is 'LIMIT'.
(Other definitions for limit that I've seen before include "end" , "Boundary, demarcation line" , "Furthest point" , "maximum number" , "qualify?" .)