Pass, slip by (6)
I believe the answer is:
elapse
I believe this is a double definition.
'pass' is the first definition.
(I know that pass can be written as elapse)
'slip by' is the second definition.
(synonyms)
(Other definitions for elapse that I've seen before include "pass in silence" , "Pass or expire - please" , "(Of time) go by" , "Tick by" , "Go by - pass away" .)