"let it slip" = revealed, usually unintentionally. So, Chandler didn't intend to tell anyone that Ross was in love with Rachel, but he accidentally did. You can't say "Chandler slipped that Ross was in love with Rachel".
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Osee
Really? May I get a comfirm from another person? Thanks.
Mr WordyOsee
Really? May I get a comfirm from another person? Thanks.
If you're referring to the anon's sentence "Chandler let slip that Ross was in love with Rachel", then yes, that's fine. The word "it" is optional.