Bank holding a sort of race (5)
I believe the answer is:
relay
'sort of race' is the definition.
'bank holding a' is the wordplay.
'bank' becomes 'rely' (I've seen this before).
'holding' means one lot of letters goes inside another.
'rely' enclosing 'a' is 'RELAY'.
(Other definitions for relay that I've seen before include "Hand, pass on" , "supplementary store of anything" , "Pass on - put down again" , "Receive and pass on - race" , "track event" .)