Observation concerning German currency (6)
I believe the answer is:
remark
'observation' is the definition.
(I know that remark is a type of observation)
'concerning german currency' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'german currency' becomes 'mark' (I've seen this in another clue).
're'+'mark'='REMARK'
(Other definitions for remark that I've seen before include "Incidental observation" , "Something said; take notice of" , "Notice or comment on" , "Mention - observe" , "what someone says?" .)