Snake found around river is dead to the world (6)
I believe the answer is:
asleep
'dead to the world' is the definition.
(I've seen this before)
'snake found around river' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'found around' is an insertion indicator.
'river' becomes 'lee' (I've seen this in other clues).
'asp' going around 'lee' is 'ASLEEP'.
'is' is the link.
(Other definitions for asleep that I've seen before include "(Of a limb) numb" , "this fast?" , "Conked out" , "Knitting up the ravelled sleeve of care" , "Snoozing" .)