Platforms some of Metro's trains used (6)
I believe the answer is:
rostra
'platforms' is the definition.
(rostrum is a kind of platform)
'some of metro's trains used' is the wordplay.
'some of' says the answer is hidden in the clue.
'ROSTRA' is hidden in the letters of 'metros trains used'.
(Other definitions for rostra that I've seen before include "stages" , "Raised platforms" , "The beaks" , "where they make their speeches?" .)