Terrible sentry, bad non-participant (9)

I believe the answer is:

bystander

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'non-participant' is the definition.
I can't judge whether this defines the answer.

'terrible sentry bad' is the wordplay.
'terrible' indicates an anagram.
'sentry'+'bad'='sentrybad'
'sentrybad' is an anagram of 'BYSTANDER'.

Can you help me to learn more?

(Other definitions for bystander that I've seen before include "Non-partcipating spectator" , "Non-participant spectator" , "Watcher of occurrence" , "Onlooker (often innocent)" , "Bad sentry may be an onlooker" .)

I've seen this clue in The Daily Express.

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