Boy's lips let slip perhaps? (8)

I believe the answer is:

possibly

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

'perhaps?' is the definition.
(I know that perhaps can be written as possibly)

'boy's lips let slip' is the wordplay.
'boy's lips' anagrams to 'POSSIBLY'.
But, I am not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for possibly that I've seen before include "Perhaps" , "Maybe" .)

I've seen this clue in The Independent.

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