Concluding provocation upset stalwarts (4,5)
I believe the answer is:
last straw
'concluding provocation' is the definition.
'last straw' can be an answer for 'provocation' (thesaurus). I'm unsure of the 'concluding' bit.
'upset stalwarts' is the wordplay.
'upset' indicates anagramming the letters.
'stalwarts' is an anagram of 'LAST STRAW'.
Can you help me to learn more?
(Other definitions for last straw that I've seen before include "Something that finally makes a situation intolerable" , "Small event finally causing a problem" , "proved back-breaking" , "Camel's back breaker" , "drinker was lucky to get?" .)