Fight the sleep one's engulfed by (6)
I believe the answer is:
resist
'fight' is the definition.
(resisting is a kind of fighting)
'the sleep one's engulfed by' is the wordplay.
'the sleep' becomes 'rest' ('rest' can be a synonym of 'sleep').
'one's' becomes 'is' (roman numeral).
'engulfed by' is an insertion indicator (I've seen this in other clues).
'rest' placed around 'is' is 'RESIST'.
(Other definitions for resist that I've seen before include "Forgo" , "Combat" , "Check; withstand" , "Stand firm against" , "Express opposition" .)