Hunting pack — they fade in marsh at regular intervals... (7)

I believe the answer is:

hyaenas

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'hunting pack' is the definition.
Although both the answer and 'pack' are nouns, they are not in the same form of the noun.

'they fade in marsh at regular intervals' is the wordplay.
'at regular intervals' means one should take alternating letters.
'they'+'fade'+'in'+'marsh'='theyfadeinmarsh'
The alternating letters of 'theyfadeinmarsh' are 'HYAENAS'.

Can you help me to learn more?

(Another definition for hyaenas that I've seen is " More than one carrion feeder".)

I've seen this clue in The Hindu.

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