One on the track of the tale, possibly? (7)
I believe the answer is:
athlete
'one on the track' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'the tale possibly?' is the wordplay.
'possibly?' is an anagram indicator (I've seen 'possibly' mean this).
'the'+'tale'='thetale'
'thetale' is an anagram of 'ATHLETE'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for athlete that I've seen before include "Sportsman in physical contests" , "Trained sportsperson" , "Eg long jumper" , "Olympian" , "Person trained to compete in sports" .)