Fight against the sleep one’s engulfed by (6)
I believe the answer is:
'fight against' is the definition.
(I've seen this in another clue)
'sleep one's engulfed by' is the wordplay.
'sleep' becomes 'rest' (synonyms).
'one's' becomes 'i's' (Roman numeral).
'engulfed by' means one lot of letters goes inside another (I've seen this in other clues).
'rest' going around 'is' is 'RESIST'.
'the' is the link.
(Other definitions for resist that I've seen before include "Bar" , "Express opposition" , "Withstand, struggle against" , "Oppose or fight against someone" , "Oppose, put up defence" .)