Lead away from party line (9)
I believe the answer is:
'lead away' is the definition.
(sidetracked means led away from something)
'party line' is the wordplay.
'party' becomes 'side' (eg a side or party in a contract).
'line' becomes 'track' (track is a kind of line).
'from' is the link.
(Other definitions for sidetrack that I've seen before include "small way" , "lead away" , "Provide diversion" , "Distract (from big issue)" , "draw away" .)