Always available as retiree to tour part of Oz (2,3)

I believe the answer is:

on tap

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'always available' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for on tap that I've seen before include "Always available" , "Constantly available like beer from keg" , "Available (for drinking?)" , "where C or H is seen?" , "Where you may see H or C" .)

I've seen this clue in The Times.

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