Bank on me only (6)
I believe the answer is:
merely
'only' is the definition.
('merely' can be a synonym of 'only')
'bank on me' is the wordplay.
'bank' becomes 'rely' (I've seen this before**).
'on' means one lot of letters go next to another.
'rely' put after 'me' is 'MERELY'.
(Other definitions for merely that I've seen before include "'Only, and nothing more (6)'" , "Just one" , "Simply - only" .)