Track series of lessons (6)
I believe the answer is:
course
This clue is a triple definition!
'track' is the first definition.
(I know that course is a more specific form of the action the track)
'series' is the second definition.
(course is a kind of series)
'of' is a link.
'lessons' is the third definition.
(a course has lessons)
(Other definitions for course that I've seen before include "Path, track" , "Curriculum" , "passage" , "soup?" , "Pursue - career - run - series of lectures" .)