... criminal with long sentence, initially they'll stay up late (5,4)

I believe the answer is:

night owls

I'm a little stuck... Click here to teach me more about this clue!

'they'll stay up late' is the definition.
I can't judge whether this defines the answer.

'criminal with long sentence initially' is the wordplay.
'criminal' indicates an anagram.
'initially' indicates taking the first letters.
The first letter of 'sentence' is 's'.
'with'+'long'='withlong'
'withlong' anagrammed gives 'nightowl'.
'nightowl'+'s'='NIGHT OWLS'

Can you help me to learn more?

(Another definition for night owls that I've seen is " Sharp-eyed types".)

I've seen this clue in The Independent.

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