Fitting move downwards when son moves out (6)

I believe the answer is:

decent

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'fitting' is the definition.
(I know that fitting can be written as decent)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for decent that I've seen before include "Moral; fitting" , "Upright" , "Morally respectable" , "Not immodest, generous" , "Kind of respectable" .)

I've seen this clue in The Times.

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