Comment about currency replaced by euro (6)
I believe the answer is:
remark
'comment' is the definition.
(a remark is a spoken comment)
'about currency replaced by euro' is the wordplay.
I cannot quite understand how this works, but
'about' could be 're' ('re' means regarding or about) and 're' is found within the answer.
'currency' could be 'mark' (I have seen 'old currency' mean 'mark' so perhaps 'currency' could also mean 'mark') and 'mark' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for remark that I've seen before include "Mention - observe" , "Written or spoken comment" , "Observation - notice" , "what someone says?" , "Say as a comment" .)