Tenacity of agent upset over separation (12)
I believe the answer is:
perseverance
'tenacity of agent' is the definition.
'perseverance' can be an answer for 'tenacity' (thesaurus). I am not sure about the 'of agent' bit.
'upset over separation' is the wordplay.
'upset over' becomes 'per' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'separation' becomes 'severance' (severance is a kind of separation).
'per'+'severance'='PERSEVERANCE'
Can you help me to learn more?
(Other definitions for perseverance that I've seen before include "Persistence, indefatigability" , "Tenacity, doggedness" , "Determination" , "It may get you through" , "Steady persistence" .)