Watch maybe as millions lodged in bank (5)
I believe the answer is:
timer
'watch maybe' is the definition.
'millions lodged in bank' is the wordplay.
'millions' becomes 'm' (abbreviation).
'lodged in' means one lot of letters goes inside another.
'bank' becomes 'tier' (both can mean a row of similar things).
'm' going into 'tier' is 'TIMER'.
'as' acts as a link.
(Other definitions for timer that I've seen before include "hourglass" , "One may have the minutes" , "Maybe watch" , "Clocklike device" , "Eg, clock" .)