Illness a youth gets in May (6)
I believe the answer is:
malady
'illness' is the definition.
(malady is a kind of health problem)
'a youth gets in may' is the wordplay.
'a youth' becomes 'lad' (I've seen this before).
'gets in' means one lot of letters goes inside another.
'lad' inserted within 'may' is 'MALADY'.
(Other definitions for malady that I've seen before include "Ailment; illness" , "Sickness or disease" , "affection" , "Disease, ailment" , "Illness or disease" .)