The US tax collection agency, not ours

I believe the answer is:

theirs

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'not ours' is the definition.
(I've seen this in another clue)

'the us tax collection agency' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'us tax' becomes 'he' (I am not sure about this - if you are sure you should believe this answer much more).
'collection agency' becomes 'irs' (I've seen this in another clue).
't'+'he'+'irs'='THEIRS'

Can you help me to learn more?

(Other definitions for theirs that I've seen before include "Owned by them" , "Belonging to those people" .)

I've seen this clue in The Daily Mail.

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