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 see how this works, but
'about' could be 're' ('re' means regarding or about) and 're' is found in the answer.
'currency' could be 'mark' (I have seen 'old currency' mean 'mark' so perhaps 'currency' could also mean 'mark') and 'mark' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
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" .)