Member on a register is a stickler for the rules (8)
I believe the answer is:
legalist
'a stickler for the rules' is the definition.
(person adhering to legal procedures)
'member on a register' is the wordplay.
'member' becomes 'leg' (leg is a kind of member).
'on' is a charade indicator (letters next to each other).
'register' becomes 'list' (listing is a kind of registering).
'leg'+'a'+'list'='LEGALIST'
'is' acts as a link.
(Other definitions for legalist that I've seen before include "One adhering to the detail of the law" , "Strict lawyer" .)