Party policy won't be one's main interest (8)
I believe the answer is:
sideline
'interest' is the definition.
(I know that sideline is a type of interest)
'party policy' is the wordplay.
'party' becomes 'side' (eg a side or party in a contract).
'policy' becomes 'line' (as in the party line).
'side'+'line'='SIDELINE'
'won't be 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 "Subsidiary trade" , "Prevent from participating" , "Secondary job" , "Boundary marker" , "not my main interest" .)