It goes round as part of the system (6)
I believe the answer is:
planet
'it goes round' is the definition.
The definition suggests a singular noun which matches the answer.
'part of the system' is the wordplay.
'part of' becomes 'pla' (I can't explain this - if you can you should believe this answer much more).
'the system' becomes 'net' (net is a kind of system).
'pla'+'net'='PLANET'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for planet that I've seen before include "Eg, Mars or Pluto" , "Celestial body that revolves around the sun" , "Earth perhaps" , "Sun's orbiter" , "Orbiting body in the heavens" .)