Crossword Genius

Constitutional member, a Labour leader (5)

Ross

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" .)

I've seen this clue in The Telegraph.
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