Spoor (5)
'spoor' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for trail that I've seen before include "Pursue" , "Drop behind" , "Beaten path through the countryside" , "Need to catch up with" , "Be losing; track" .)
(Other definitions for scent that I've seen before include "Get wind of" , "Fragrance" , "Pleasant odour" , "Perfume for the tracker" , "Trail" .)
(Other definitions for track that I've seen before include "Course over which races are run" , "song off a record" , "Lines" , "Follow < course" , "Part of record" .)