Just bank on reversal of rating (6)
I believe the answer is:
barely
'just' is the definition.
(synonyms)
'bank on reversal of rating' is the wordplay.
'bank' becomes 'rely' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'reversal of' shows that the letters should be reversed in order.
'rating' becomes 'ab'.
'ab' back-to-front is 'ba'.
'rely' put after 'ba' is 'BARELY'.
(Other definitions for barely that I've seen before include "By a small margin" , "Hardly" , "how babies are born?" , "how naturists exist" , "as Godiva rode?" .)