Rate halved with bank not very often (6)
I believe the answer is:
rarely
'not very often' is the definition.
(I've seen this before)
'rate halved with bank' is the wordplay.
'halved' suggests halving the letters.
'with' means one lot of letters go next to another.
'bank' becomes 'rely' (I've seen this before).
'rate' cut in half is 'ra'.
'ra'+'rely'='RARELY'
(Other definitions for rarely that I've seen before include "Seldom, not frequently" , "Hardly ever, most infrequently" , "Not done or occurring very often" , "very well" , "once in a blue moon" .)