Breathe out: breathe your last? (6)
I believe the answer is:
expire
I believe this clue is a double definition.
'breathe out' is the first definition.
(I know that breathe out can be written as expire)
'breathe your last?' is the second definition.
(Other definitions for expire that I've seen before include "Expel air or die" , "Breathe out or run out" , "Breathe out - die" , "Lose validity" , "Die, give out" .)