Month in which boy makes complaint (6)
I believe the answer is:
malady
'complaint' 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' means one lot of letters goes inside another.
'boy' becomes 'lad' (informal term for boy).
'may' enclosing 'lad' is 'MALADY'.
'makes' acts as a link.
(Other definitions for malady that I've seen before include "Sickness, ailment" , "hay fever, perhaps" , "Disease, ailment" , "Ailment; illness" , "Disease or disorder" .)