Complaint of youth in spring (6)
I believe the answer is:
malady
'complaint' is the definition.
(thesaurus)
'youth in spring' is the wordplay.
'youth' becomes 'lad' (I've seen this before).
'in' indicates putting letters inside.
'spring' becomes 'may' (I've seen this before).
'lad' placed inside 'may' is 'MALADY'.
'of' is the link.
(Other definitions for malady that I've seen before include "Disease or disorder" , "hay fever, perhaps" , "Sickness, ailment" , "Complaint" , "Illness or disease" .)