Coming round motorway, knock Pat down (4)
I believe the answer is:
tamp
'pat down' is the definition.
(I've seen this in another clue)
'coming round motorway knock' is the wordplay.
'coming round' means one lot of letters goes inside another.
'motorway' becomes 'M' (as in motorway names - Mt, M4 etc).
'knock' becomes 'tap' (synonyms).
'm' going within 'tap' is 'TAMP'.
(Other definitions for tamp that I've seen before include "press down firmly" , "Press down tightly" , "Compact" , "Pack or ram down tightly" , "Pack, force down" .)