Search and rescue by operations leader into nasty gasses across large part of the North Atlantic (8,3)
I believe the answer is:
sargasso sea
'large part of the north atlantic' is the definition.
(part of the Atlantic Ocean)
'search and rescue by operations leader into nasty gasses across' is the wordplay.
I cannot quite understand how this works, but
'leader' could be 'e' (I've seen this in other clues) and 'e' is found in the answer.
'gasses' could be 'os' (O is an example) and 'os' is located in the answer.
'search and rescue' could be 'sar' (abbreviation) and 'sar' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for sargasso sea that I've seen before include "Part of N. Atlantic where weed gathers" , "where one may be becalmed" , "weed-filled waters" , "Weedy part of Atlantic Ocean" , "Weed-covered part of Atlantic Ocean" .)