Run on a bank -- once in a blue moon (6)
I believe the answer is:
rarely
'once in a blue moon' is the definition.
(I've seen this in another clue)
'run on a bank' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'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" , "Not very often, seldom in fact" , "Hardly ever, most infrequently" , "Seldom, not frequently" , "Very occasionally" .)