Complaint of youth in May (6)
I believe the answer is:
malady
'complaint' is the definition.
(malady is a kind of health problem)
'youth in may' is the wordplay.
'youth' becomes 'lad' (lad means a young man).
'in' is an insertion indicator.
'lad' inserted into 'may' is 'MALADY'.
'of' is the link.
(Other definitions for malady that I've seen before include "affection" , "Illness or disease" , "hay fever, perhaps" , "Afflication" , "Disease, ailment" .)