One behind the other in metropolitan demonstration (6)
I believe the answer is:
tandem
'one behind the other' is the definition.
(synonyms)
'in metropolitan demonstration' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TANDEM' is hidden in the letters of 'metropolitan demonstration'.
(Other definitions for tandem that I've seen before include "Two-seater bicycle" , "One behind another, as on bicycle" , "Two-wheeler for two" , "Bicycle made for two" , "Two-seater bike" .)