Party policy not one's main interest (8)
I believe the answer is:
sideline
'interest' is the definition.
(sideline is a kind of interest)
'party policy' is the wordplay.
'party' becomes 'side' (eg a side or party in a contract).
'policy' becomes 'line' (the party line is a party's policy on something).
'side'+'line'='SIDELINE'
'not one's main' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sideline that I've seen before include "Activity outside one's main job" , "Subsidiary activity" , "Remove from the centre of activity" , "Secondary activity" , "Boundary marker" .)