Working together (13)
(Another definition for collaborating that I've seen is " Jointly working".)
'working together' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for in tandem that I've seen before include "One behind the other or alongside" , "'One behind the other, as on a bicycle (2,6)'" , "Working together" , "Manned it as twosome" , "Together - as on a bicycle made for two" .)
(Another definition for as one that I've seen is " Totally united".)