Two-seat bicycle (6)
I believe the answer is:
tandem
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">two-seat bicycle</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)