Morons laugh tactlessly seizing offensive (9)

I believe the answer is:

onslaught

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'offensive' is the definition.
(thesaurus)

'morons laugh tactlessly seizing' is the wordplay.
'ONSLAUGHT' is hidden within the letters.
However, I am unsure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for onslaught that I've seen before include "offensive action" , "Heavy attack" , "Hunt goals (anag) - offensive" , "Destructive attack" , "Fierce attack" .)

I've seen this clue in The Telegraph.

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