Final flag hoisted above ship's stern, done in desperation at the eleventh hour (4-4)
I believe the answer is:
last-gasp
'final' is the definition.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for last-gasp that I've seen before include "Finalk expiration, moment of death" , "Done in desperation at the final moment" , "'Final breath (4,4)'" , "Final exhalation before death" , "Final breath or done at the last possible moment of effort" .)