Deal with leader of youth convention (6)
I believe the answer is:
treaty
'convention' is the definition.
(thesaurus)
'deal with leader of youth' is the wordplay.
'deal with' becomes 'treat' (synonyms).
'leader of' says to take the initial letters.
The initial letter of 'youth' is 'y'.
'treat'+'y'='TREATY'
(Other definitions for treaty that I've seen before include "'Pact, accord (6)'" , "Compact; concordat" , "Political or private agreement" , "Written agreement between two states" , "Yatter (anag.)" .)