MP's constituent (6)
I believe the answer is:
member
I believe this is a double definition.
'mp's' is the first definition.
(I've seen this before)
'constituent' is the second definition.
(I've seen this before)
(Other definitions for member that I've seen before include "Person belonging to a particular club or society" , "Elected person" , "As an example, arm" , "Limb - one that belongs" , "One belongs to a society" .)