Doctor let ma in for illness (7)
I believe the answer is:
ailment
'for illness' is the definition.
(an ailment is an illness or health problem)
'doctor let ma in' is the wordplay.
'doctor' is an anagram indicator.
'let'+'ma'+'in'='letmain'
'letmain' anagrammed gives 'AILMENT'.
(Other definitions for ailment that I've seen before include "I lament (anag.)" , "Bodily disorder or disease" , "Physical disorder" , "Tim Alen has an odd indisposition." , "Minor illness" .)