Son in fairly good journey down mountain? (7)

I believe the answer is:

descent

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'journey down mountain?' is the definition.
'descent' can be an answer for 'journey' (I have seen 'Downward journey ' mean 'descent' so perhaps 'journey' could also mean 'descent'). I am not certain of the 'down mountain?' bit.

'son in fairly good' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'in' means one lot of letters goes inside another.
'fairly good' becomes 'decent' (I've seen this before).
's' put inside 'decent' is 'DESCENT'.

Can you help me to learn more?

(Other definitions for descent that I've seen before include "Coming down" , "line" , "Ancestry - downward move" , "Drop" , "Lineage" .)

I've seen this clue in The Times.

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