Bicycle for two (6)
I believe the answer is:
tandem
'bicycle for two' 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" , "Two-seater bike" , "Two-person bike" , "Two-seater bicycle" , "One behind another, as on bicycle" .)