Road almost completely overwhelmed (5)
I believe the answer is:
route
'road' is the definition.
(synonyms)
'almost completely overwhelmed' is the wordplay.
'almost completely' means to remove the last letter.
'overwhelmed' becomes 'routed' (synonyms).
'routed' with its last letter taken off is 'ROUTE'.
(Other definitions for route that I've seen before include "Course followed" , "track" , "Choice of roads in an itinerary" , "Course to destination" , "Course to be travelled" .)