Constitutional member, a Labour leader (5)
I believe the answer is:
legal
'constitutional' is the definition.
(thesaurus)
'member a labour leader' is the wordplay.
'member' becomes 'leg' (I've seen this before).
'leader' suggests taking the first letters.
The first letter of 'labour' is 'l'.
'leg'+'a'+'l'='LEGAL'
(Other definitions for legal that I've seen before include "right" , "not banned" , "Observing acts" , "Authorised" , "Lawful" .)