Watch when bank grabs money! (5)
I believe the answer is:
timer
'watch' is the definition.
(I've seen this before)
'bank grabs money' is the wordplay.
'bank' becomes 'tier' (both can mean a row of similar things).
'grabs' indicates putting letters inside.
'money' becomes 'm' (abbreviation in economics).
'tier' placed around 'm' is 'TIMER'.
'when' is the link.
(Other definitions for timer that I've seen before include "Clock or watch" , "hourglass" , "Eg, clock" , "Maybe watch" , "Clocklike actuator" .)