Nincompoop concealing past that's deeply alarming (5)

I believe the answer is:

abyss

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'deeply alarming' is the definition.
I can't tell whether this defines the answer.

'nincompoop concealing past' is the wordplay.
'nincompoop' becomes 'ass' (I've seen this before).
'concealing' is an insertion indicator.
'past' becomes 'by' (as in years gone by).
'ass' enclosing 'by' is 'ABYSS'.

'that's' acts as a link.

Can you help me to learn more?

(Other definitions for abyss that I've seen before include "deep depression" , "Bottomless gulf" , "crevasse" , "ocean depth" , "Deep hole, seems bottomless" .)

I've seen this clue in The Times.

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