Callous conservative becomes good, providing basic sustenance (5)

I believe the answer is:

gruel

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'sustenance' is the definition.
(gruel is a kind of sustenance)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for gruel that I've seen before include "Thin oatmeal dish" , "Thin liquid food" , "Thin food of boiled oatmeal" , "cheap fare" , "Luger (anag)" .)

I've seen this clue in The Irish Times.

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