Crossword Genius

Impound weapon to catch single villain in Gotham City (7)

Ross

I believe the answer is:

penguin

I'm a little stuck... Click here to teach me more about this clue!

'villain in gotham city' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'impound weapon to catch single' is the wordplay.
'impound' becomes 'pen' (I can't justify this - if you can you should give a lot more credence to this answer).
'weapon' becomes 'gun' (gun is a kind of weapon).
'to catch' means one lot of letters goes inside another.
'single' becomes 'I' (one).
'gun' placed around 'i' is 'guin'.
'pen'+'guin'='PENGUIN'

Can you help me to learn more?

(Other definitions for penguin that I've seen before include "Waddling bird" , "Flightless marine bird" , "Swimming bird" , "Black-and-white Arctic bird" , "King's an example" .)

I've seen this clue in the Sydney Morning Herald.
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