Well-informed Labour leader's merited (7)
I believe the answer is:
learned
'well-informed' is the definition.
(thesaurus)
'labour leader's merited' is the wordplay.
'leader' says to take the initial letters.
'merited' becomes 'earned' (I've seen this before).
The initial letter of 'labour' is 'l'.
'l'+'earned'='LEARNED'
(Other definitions for learned that I've seen before include "Studied" , "Educated, scholarly" , "Intellectual" , "Highly educated" , "Erudite - academic" .)