Lack of vitality is not so much about one elected member (northern) (8)
I believe the answer is:
limpness
'lack of vitality' is the definition.
(limp can mean lacking vitality)
'not so much about one elected member northern' is the wordplay.
'not so much' becomes 'less' (I've seen this in other clues).
'about' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'elected member' becomes 'MP' (Member of Parliament).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'i'+'mp'+'n'='impn'
'less' placed around 'impn' is 'LIMPNESS'.
'is' is the link.
(Another definition for limpness that I've seen is " Lack of stiffness".)