Just bank on Mike losing heart (6)
I believe the answer is:
merely
'just' is the definition.
('merely' can be a synonym of 'just')
'bank on mike losing heart' is the wordplay.
'bank' becomes 'rely' (I've seen this before**).
'on' is a charade indicator (letters next to each other).
'losing heart' means to remove the middle letters.
'mike' with its middle taken out is 'me'.
'rely' after 'me' is 'MERELY'.
(Other definitions for merely that I've seen before include "'Only, and nothing more (6)'" , "Just one" , "Simply - only" .)