Crossword Genius

Seat of second party (8)

Ross

I believe the answer is:

backside

'seat' is the definition.
('backside' can be a synonym of 'seat')

'second party' is the wordplay.
'second' becomes 'back' (to second a motion is to back it).
'party' becomes 'side' (I've seen this before).
'back'+'side'='BACKSIDE'

'of' acts as a link.

(Other definitions for backside that I've seen before include "Buttocks" , "Rear" , "Bottom" , "Behind" , "Hind part" .)

I've seen this clue in The Independent.
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