Transport used by some in metropolitan demography (6)
I believe the answer is:
tandem
'transport' is the definition.
(tandem is a kind of transport)
'some in metropolitan demography' is the wordplay.
'some in' indicates the answer is hidden within the clue.
'TANDEM' is hidden in the letters of 'metropolitan demography'.
'used by' acts as a link.
(Other definitions for tandem that I've seen before include "vehicle" , "Bike for two cyclists" , "it's not normally what one rides" , "Bicycle-built-for-two" , "Bicycle for two riders" .)