Route incorporating avenue is a slog (7)
I believe the answer is:
travail
'a slog' is the definition.
(thesaurus)
'route incorporating avenue' is the wordplay.
'route' becomes 'trail' (trail is a kind of route).
'incorporating' indicates putting letters inside.
'avenue' becomes 'av' (short for avenue).
'trail' going around 'av' is 'TRAVAIL'.
'is' is the link.
(Other definitions for travail that I've seen before include "Labour, toil" , "Old word for pangs of childbirth" , "In old use, the pains of childbirth" , "Use of physical or mental energy; excessive labour..." , "Exertion, elbow grease" .)