Always veer off course, occasionally crossing motorway (8)
I believe the answer is:
evermore
'always' is the definition.
(I've seen this before)
'veer off course occasionally crossing motorway' is the wordplay.
'off' indicates anagramming the letters.
'occasionally' means one should take alternating letters.
'crossing' indicates putting letters inside.
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
The alternating letters of 'course' are 'ore'.
'veer' with letters rearranged gives 'ever'.
'ever'+'ore'='everore'
'everore' placed around 'm' is 'EVERMORE'.
(Other definitions for evermore that I've seen before include "unceasingly" , "(For) perpetuity" , "Always, forever" , "Always and without end" , "For which, permanently" .)