No way back when oxygen fails? What a bore (4)
I believe the answer is:
yawn
'a bore' is the definition.
(I've seen this before)
'no way back when oxygen fails?' is the wordplay.
I cannot quite see how this works, but
'no' could be 'n' (common abbreviation y/n) and 'n' is located in the answer.
an anagram of 'way' is 'yaw' which is located in the answer.
No letters remain.
This explanation may well be incorrect...
'what' acts as a link.
(Other definitions for yawn that I've seen before include "Involuntary sign of tiredness or boredom" , "Intake of breath through a wide-open mouth" , "Bored sign" , "Bore" , "Oscitate" .)