Switching device ace in bank (5)
I believe the answer is:
relay
'switching device' is the definition.
'Relay' can be an answer for 'device' (relay is a kind of device). I'm not sure about the 'switching' bit.
'ace in bank' is the wordplay.
'ace' becomes 'a' (abbreviation).
'in' is an insertion indicator.
'bank' becomes 'rely' (to bank on is to rely on).
'a' put inside 'rely' is 'RELAY'.
Can you help me to learn more?
(Other definitions for relay that I've seen before include "Team of workers" , "Broadcast" , "Hand, pass on" , "track event" , "article in bank" .)