Nothing more than a 50-50 chance nevertheless (4,2)
I believe the answer is:
even so
'nevertheless' is the definition.
(synonyms)
'nothing more than a 50-50 chance' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'more than' is a charade indicator (letters next to each other).
'a 50-50 chance' becomes 'evens' (I've seen this before).
'o' put after 'evens' is 'EVEN SO'.
(Other definitions for even so that I've seen before include "Nevertheless, albeit" , "anyway" , "Notwithstanding" , "however" , "Despite that, nevertheless" .)