In area of influence old Republican was effective (5)
I believe the answer is:
orbit
'in area of influence' is the definition.
I can't judge whether this defines the answer.
'old republican was effective' is the wordplay.
'old' becomes 'o' (abbreviation).
'republican' becomes 'R' (abbreviation).
'was effective' becomes 'bit' (bite can mean to have an effect or impact).
'o'+'r'+'bit'='ORBIT'
Can you help me to learn more?
(Other definitions for orbit that I've seen before include "Path of planet etc" , "Revolve round" , "Sphere of activity" , "astronauts' course" , "Planetary path" .)