Ain't my checks free? One's on the house! (7,5)

I believe the answer is:

chimney stack

'one's on the house' is the definition.
(a chimney stack is found on the roof of a house)

'ain't my checks free?' is the wordplay.
'free?' indicates an anagram (I've seen 'freed' mean this).
'aint'+'my'+'checks'='aintmychecks'
'aintmychecks' is an anagram of 'CHIMNEY-STACK'.

(Another definition for chimney stack that I've seen is " Roof flue structure".)

I've seen this clue in The Times.

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