Regular course in operational research took effect (5)
I believe the answer is:
orbit
'regular course' is the definition.
(eg regular route around the sun)
'operational research took effect' is the wordplay.
'operational research' becomes 'OR' (abbreviation in science).
'took effect' becomes 'bit' (bite can mean to have an effect).
'or'+'bit'='ORBIT'
'in' is the link.
(Other definitions for orbit that I've seen before include "Circuit of a planet" , "Track of satellite" , "Sun's circulation" , "Circumnavigate" , "Planet's path" .)