Observation concerning former German currency (6)
I believe the answer is:
remark
'observation' is the definition.
(synonyms)
'concerning former german currency' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'former german currency' becomes 'mark' (I've seen this before).
're'+'mark'='REMARK'
(Other definitions for remark that I've seen before include "Say as a comment" , "Notice or comment on" , "Make mention of" , "what someone says?" , "Comment, observation" .)