Worry former partner with messy separation (12)
I believe the answer is:
exasperation
'worry' is the definition.
(thesaurus)
'former partner with messy separation' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'with' is a charade indicator (letters next to each other).
'messy' indicates an anagram.
'separation' anagrammed gives 'asperation'.
'ex'+'asperation'='EXASPERATION'
(Other definitions for exasperation that I've seen before include "Fury" , "Annoyance" , "Disgruntlement" , "Intense irritation" .)