Bypass over motorway heading for tunnel (4)
I believe the answer is:
omit
'bypass' is the definition.
(I've seen this in another clue)
'over motorway heading for tunnel' is the wordplay.
'over' becomes 'o' (cricket abbreviation).
'motorway' becomes 'MI' (resembles M1, English motorway).
'heading for' says to take the initial letters.
The first letter of 'tunnel' is 't'.
'o'+'mi'+'t'='OMIT'
(Other definitions for omit that I've seen before include "Neglect to include" , "Leave out, do not include (4)" , "Leave out, perhaps in error" , "Exclude; forget to include" , "Leave out or forget" .)