One gets to slide during sleep? It's most hazardous (8)
I believe the answer is:
riskiest
'most hazardous' is the definition.
('risky' can be a synonym of 'hazardous')
'one gets to slide during sleep?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'gets' says to put letters next to each other.
'to slide' becomes 'ski' (I've seen this before).
'during' means one lot of letters goes inside another.
'sleep?' becomes 'rest' (sleeping is a kind of resting**).
'i'+'ski'='iski'
'iski' inserted within 'rest' is 'RISKIEST'.
'it's' is the link.
(Another definition for riskiest that I've seen is " Most dangerous".)