Mile in excellent fast time leading to illness (7)
I believe the answer is:
ailment
'illness' is the definition.
(thesaurus)
'mile in excellent fast time leading' is the wordplay.
'mile' becomes 'm' (common abbreviation).
'in' indicates putting letters inside.
'excellent' becomes 'ai'.
'fast time' becomes 'lent' (I've seen this before).
'leading' says to put letters next to each other.
'ai'+'lent'='ailent'
'm' placed inside 'ailent' is 'AILMENT'.
'to' is the link.
(Other definitions for ailment that I've seen before include "problem" , "Slight but persistent illness" , "I lament (anag.)" , "Malady" , "Physical disorder" .)