Two-seat bicycle (6)
I believe the answer is:
tandem
'two-seat 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 with two saddles" , "Bicycle made for two" , "Bicycle for two riders" , "Two-wheeler for two" , "Bicycle built for two" .)