Month in which boy gets an illness (6)
I believe the answer is:
malady
'an illness' is the definition.
(malady is a kind of health problem)
'month in which boy' is the wordplay.
'month' becomes 'may' (May is an example).
'in which' indicates putting letters inside.
'boy' becomes 'lad' (lad is a kind of boy).
'may' enclosing 'lad' is 'MALADY'.
'gets' acts as a link.
(Other definitions for malady that I've seen before include "Ailment; illness" , "Sickness or disease" , "Sickness, ailment" , "hay fever, perhaps" , "Complaint" .)