One behind another (2,6)
I believe the answer is:
in tandem
'one behind another' is the definition.
This is the entire clue.
(Other definitions for in tandem that I've seen before include "Manned it as twosome" , "Together (as a pair)" , "Working together" , "'One behind the other, as on a bicycle (2,6)'" , "One behind the other or alongside" .)