Illness making Tim lean badly (7)
I believe the answer is:
ailment
'illness' is the definition.
(an ailment is an illness or health problem)
'tim lean badly' is the wordplay.
'badly' indicates anagramming the letters (letters in the wrong order).
'tim'+'lean'='timlean'
'timlean' is an anagram of 'AILMENT'.
'making' is the link.
(Other definitions for ailment that I've seen before include "Slight but persistent illness" , "Malady" , "Complaint, affliction" , "problem" , "I lament (anag.)" .)