Just in case
I believe the answer is:
lest
'just in case' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lest that I've seen before include "To avoid the risk that - we forget?" , "For fear that (something should happen)" , "To avoid the risk that - let's do it" , "In order not to, say forget?" , "For fear that (something happens)" .)