Rate halved with bank not very often (6)
I believe the answer is:
Here is my best explanation:
'not very often' is the definition.
'rate halved with bank' is the subsidiary indication.
'halved' suggests halving the letters.
'with' is a charade indicator (letters next to each other).
'bank' becomes 'rely'.
'rate' cut in half is 'ra'.
(Other definitions for rarely that I've seen before include "It's not often", "very well", "Seldom, not frequently", "Not very often, seldom in fact", "Very occasionally", "Hardly ever, most infrequently", "Very infrequently", "when hundred-to-one horse wins?", "once in a blue moon" and "Gas used in fluorescent signs".)