They make an impression by putting Rontgen into unstable nerve gas (9)
I believe the answer is:
engravers
'rontgen into unstable nerve gas' is the definition.
I can't tell whether this defines the answer.
'they make an impression by' is the wordplay.
'they' becomes 'rs' (I am not sure about this - if you are sure you should believe this answer much more).
'make an impression' becomes 'engrave' (I've seen this before).
'by' means one lot of letters go next to another.
'rs' put after 'engrave' is 'ENGRAVERS'.
'putting' is the link.
Can you help me to learn more?
(Another definition for engravers that I've seen is " They impress".)