One during restless sleep to slip away (6)
I believe the answer is:
elapse
'slip away' is the definition.
(I've seen this before)
'one during restless sleep' is the wordplay.
'one' becomes 'a'.
'during' means one lot of letters goes inside another.
'restless' indicates an anagram (letters not resting).
'sleep' with letters rearranged gives 'elpse'.
'a' placed within 'elpse' is 'ELAPSE'.
'to' acts as a link.
(Other definitions for elapse that I've seen before include "Slip away" , "Passing" , "Pass or expire - please" , "Go past" , "run by" .)