Until now, he always presumed pessimistic youth leaders were depressed (7)
I believe the answer is:
unhappy
'depressed' is the definition.
(thesaurus)
'until now he always presumed pessimistic youth leaders' is the wordplay.
'leaders' suggests taking the first letters.
The initial letters of 'until now he always presumed pessimistic youth' is 'UNHAPPY'.
'were' acts as a link.
(Other definitions for unhappy that I've seen before include "Not fortunate" , "In low spirits" , "Dejected, not contented" , "Woeful" , "Sad, distressed" .)