Annual return adopted by bank on only a few occasions (6)
I believe the answer is:
Here is my best explanation:
'only a few occasions' is the definition.
I can't judge whether this definition defines the answer.
'annual return adopted by bank' is the subsidiary indication.
'annual return' becomes 'ar' (this might be a standard abbreviation of which I'm unaware).
'adopted by' is an insertion indicator (I've seen this in other clues).
'bank' becomes 'rely' (I've seen this before).
'ar' inserted within 'rely' is 'rarely'.
'on' is the link.
(Other definitions for rarely that I've seen before include "Very infrequently", "when hundred-to-one horse wins?", "Not done or occurring very often", "very well", "Gas used in fluorescent signs", "Seldom, not often", "Very occasionally", "It's not often", "Infrequently" and "Hardly ever, most infrequently".)