One behind the other (6)
I believe the answer is:
tandem
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one behind the other</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for tandem that I've seen before include "Two-seater bike" , "Two wheels for two" , "Bicycle-built-for-two" , "Two-wheeler for two" , "Two-seat bicycle" .)