Lag behind on the track (5)
I believe the answer is:
trail
I believe this is a double definition.
'lag behind' is the first definition.
(thesaurus)
'the track' is the second definition.
(trail is a kind of track)
'on' acts as a link.
(Other definitions for trail that I've seen before include "Drag behind" , "Track or mark left by something that has passed" , "Fall behind" , "Track or scent used in following someone" , "Straggle" .)