Genuine member's first field of interest (5)
I believe the answer is:
realm
'field of interest' is the definition.
(I've seen this in another clue)
'genuine member's' is the wordplay.
'genuine' becomes 'real' (similar in meaning).
'member' becomes 'm' (political abbreviation).
'real'+'m'='REALM'
'first' is the link.
(Other definitions for realm that I've seen before include "Area of interest/activity" , "Knowledge base" , "Kingdom could be lamer" , "region" , "monarch's responsibility" .)