Among the ten runners, mine is leading (7)
I believe the answer is:
eminent
'is leading' is the definition.
(thesaurus)
'among the ten runners mine' is the wordplay.
'among' means one lot of letters goes inside another (I've seen this in other clues).
'the ten runners' becomes 'ent' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ent' enclosing 'mine' is 'EMINENT'.
Can you help me to learn more?
(Other definitions for eminent that I've seen before include "Respected; distinguished" , "Respected; notable" , "Illustrious like cardinal?" , "High" , "Important" .)