Holiday telegram could cause alarm (4,4)

I believe the answer is:

trip wire

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'cause alarm' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?

'holiday telegram' is the wordplay.
'holiday' becomes 'trip' (I've seen this in another clue).
'telegram' becomes 'wire' (synonyms).
'trip'+'wire'='TRIP WIRE'

'could' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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