Reveal by accident
'reveal by accident' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for let slip that I've seen before include "Fail to keep Secret" , "Allow to escape (information etc)" , "Allow to fall or reveal a secret accidentally" , "Inadvertently reveal (something)" , "accidentally revealed" .)
'reveal by accident' is the definition.
(I've seen this in another clue)
This is all the clue.