Subterranean passage in progress (5,3)

I believe the answer is:

under way

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'in progress' is the definition.
(I've seen this before)

'subterranean passage' is the wordplay.
'subterranean' becomes 'under' (I can't explain this - if you can you should believe this answer much more).
'passage' becomes 'way' (I've seen this in other clues).
'under'+'way'='UNDER WAY'

Can you help me to learn more?

(Other definitions for under way that I've seen before include "Tunnel" , "In motion (nautical)" , "Going" , "Might sewer network be" , "In progress" .)

I've seen this clue in The Times.

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