Boy seen during month of illness (6)
I believe the answer is:
malady
'illness' is the definition.
(I know that illness can be written as malady)
'boy seen during month' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'seen during' is an insertion indicator.
'month' becomes 'may' (May is an example).
'lad' placed inside 'may' is 'MALADY'.
'of' acts as a link.
(Other definitions for malady that I've seen before include "affection" , "Complaint" , "Illness or disease" , "Disease or disorder" , "Disease, ailment" .)