Entirely free of error, yet merely passable! (3,5)
I believe the answer is:
all right
'yet merely passable' is the definition.
'all right' can be an answer for 'passable' (I've seen this before). I am not sure about the 'yet merely' bit.
'entirely free of error' is the wordplay.
'entirely' becomes 'all' (I've seen this in another clue).
'free of error' becomes 'right' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'all'+'right'='ALL-RIGHT'
Can you help me to learn more?
(Other definitions for all right that I've seen before include "Very well, I agree" , "Agreed" , "Very well, whatever you say" , "Allowable; fairly well" , "never sinister?" .)