Regrettably intermixes as a last resort (2,8)
I believe the answer is:
in extremis
'as a last resort' is the definition.
I can't judge whether this defines the answer.
'regrettably intermixes' is the wordplay.
'regrettably' is an anagram indicator.
'intermixes' is an anagram of 'IN EXTREMIS'.
Can you help me to learn more?
(Other definitions for in extremis that I've seen before include "At the point of death" , "Facing death" , "at a desperate point" , "as last resort" , "dying" .)