Good to replace surface of European coin once (5)
I believe the answer is:
groat
'coin once' is the definition.
'groat' can be an answer for 'coin' (I have seen 'Old coin ' mean 'groat' so perhaps 'coin' could also mean 'groat'). I am not sure about the 'once' bit.
'good to replace surface of european' is the wordplay.
I cannot really see how this works, but
'good' could be 'g' (abbreviation) and 'g' is found in the answer.
'of' could be 'o' and 'o' is found in the answer.
The remaining letters 'rat' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for groat that I've seen before include "Fourpenny piece" , "Old English silver coin" , "Old four-pence coin" , "Old coin worth four pence" , "Old English coin worth four pennies" .)