Crossword Genius

They supply brewed drinks on tap (3,4)

Ross

I believe the answer is:

tea urns

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

'they supply brewed' is the definition.
The definition suggests an adverb but the answer is not.

'drinks on tap' is the wordplay.
'drinks' becomes 'teas' (tea is a kind of drink).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'tap' becomes 'urn' (I can't justify this - if you can you should believe this answer much more).
'teas' enclosing 'urn' is 'TEA-URNS'.

Can you help me to learn more?

(Other definitions for tea urns that I've seen before include "refreshments they provide" , "containers" .)

I've seen this clue in The Globe and Mail.
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