Seldom right, a bank (6)
I believe the answer is:
rarely
'seldom' is the definition.
(I know that seldom can be written as rarely)
'right a bank' is the wordplay.
'right' becomes 'r' (common abbreviation).
'bank' becomes 'rely' (to bank on something is to rely on it).
'r'+'a'+'rely'='RARELY'
(Other definitions for rarely that I've seen before include "Gas used in fluorescent signs" , "very well" , "when hundred-to-one horse wins?" , "Very occasionally" , "once in a blue moon" .)