Totally one-sided, but that's OK (3,5)
I believe the answer is:
all right
'that's ok' is the definition.
(synonyms)
'totally one-sided' is the wordplay.
'totally' becomes 'all'.
'one sided' becomes 'right' (I am not sure about this - if you are sure you should believe this answer much more).
'all'+'right'='ALL-RIGHT'
'but' is the link.
Can you help me to learn more?
(Other definitions for all right that I've seen before include "no mistake!" , "Very well, whatever you say" , "Very well, I agree" , "Safe - acceptable" , "unharmed?" .)