One in congress bottling it but making an impression (9)
I believe the answer is:
imitating
'making an impression' is the definition.
I can't tell whether this defines the answer.
'one in congress bottling it' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in congress' becomes 'mating' (I can't justify this - if you can you should give a lot more credence to this answer).
'bottling' means one lot of letters goes inside another (bottling is a kind of containing).
'mating' placed around 'it' is 'mitating'.
'i'+'mitating'='IMITATING'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for imitating that I've seen before include "miming" , "Copying" .)