Running across a motorway south of Lincoln? (5)
I believe the answer is:
abeam
'running across' is the definition.
(nautical term)
'a motorway south of lincoln?' is the wordplay.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'south of' is a charade indicator (letters next to each other) (in a down clue, some letters go under others).
'lincoln?' becomes 'abe' (Abraham Lincoln).
'a'+'m'='am'
'am' after 'abe' is 'ABEAM'.
(Other definitions for abeam that I've seen before include "going from one side to the other" , "across the deck" , "across width of ship" , "across centre" , "The way sailor goes across ship" .)