Complaint might include boy (6)
I believe the answer is:
malady
'complaint' is the definition.
(thesaurus)
'might include boy' is the wordplay.
'might' becomes 'may' (I've seen this before).
'include' means one lot of letters goes inside another.
'boy' becomes 'lad' (lad is a kind of boy).
'may' enclosing 'lad' is 'MALADY'.
(Other definitions for malady that I've seen before include "Sickness or disease" , "Disease or disorder" , "Disease, ailment" , "Afflication" , "affection" .)