Two-seater bicycle (6)
I believe the answer is:
tandem
'two-seater bicycle' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tandem that I've seen before include "Bike for two cyclists" , "Two-seater bicycle" , "it's not normally what one rides" , "Bike with two saddles" , "Two-seater bike" .)