bicycle built for two (6)
I believe the answer is:
tandem
'bicycle built for two' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tandem that I've seen before include "Bicycle built for two" , "One behind another, as on bicycle" , "Two-person cycle" , "Two-seater bike" , "Bicycle for two riders" .)