Artist going to bank once in a blue moon (6)
I believe the answer is:
Here is my best explanation:
'once in a blue moon' is the definition.
'artist going to bank' is the subsidiary indication.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'going to' says to put letters next to each other.
'bank' becomes 'rely'.
(Other definitions for rarely that I've seen before include "Not very often, seldom in fact", "when hundred-to-one horse wins?", "Very infrequently", "once in a blue moon", "Gas used in fluorescent signs", "Not done or occurring very often", "Seldom, not often", "Infrequently", "It's not often" and "very well".)