Second token used in Montana (6)
I believe the answer is:
moment
'second' is the definition.
(I know that second can be written as moment)
'token used in montana' is the wordplay.
'token' becomes 'omen' (both can mean a sign or indication of something).
'used in' indicates putting letters inside.
'montana' becomes 'mt' (abbreviation).
'omen' inserted inside 'mt' is 'MOMENT'.
(Other definitions for moment that I've seen before include "little while" , "Importance - point in time" , "An instant in time - important" , "Second, flash" , "Brief period of time" .)