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