Totally one-sided, but that's OK (3,5)
I believe the answer is:
all right
'ok' is the definition.
(synonyms)
'totally one-sided but' is the wordplay.
'totally' becomes 'all' (I've seen this in another clue).
'one sided but' becomes 'right' (I can't explain this - if you can you should give a lot more credence to this answer).
'all'+'right'='ALL-RIGHT'
'that's' acts as a 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?" .)