Lethal grin about long session (3,7)

I believe the answer is:

all nighter

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'long session' is the definition.
'all nighter' can be an answer for 'session' (I have seen 'Extended session' mean 'all nighter' so perhaps 'session' could also mean 'all nighter'). I am unsure of the 'long' bit.

'lethal grin about' is the wordplay.
'about' indicates anagramming the letters.
'lethal'+'grin'='lethalgrin'
'lethalgrin' anagrammed gives 'ALL-NIGHTER'.

Can you help me to learn more?

(Other definitions for all nighter that I've seen before include "Shift, perhaps" , "pop concert, say" , "Extended session" .)

I've seen this clue in The Daily Express.

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