Unusually large seat for each (6)
I believe the answer is:
bumper
'unusually large' is the definition.
(I've seen this before)
'seat for each' is the wordplay.
'seat' becomes 'bum' (synonyms).
'for each' becomes 'per' (per person).
'bum'+'per'='BUMPER'
(Other definitions for bumper that I've seen before include "jumbo" , "Something big" , "Very large - vehicle protector" , "Big - fender" , "fender in US" .)