This person's upset and cross -- it's a mess (3)
I believe the answer is:
mix
'a mess' is the definition.
(thesaurus)
'this person's upset and cross' is the wordplay.
'this person's' becomes 'im' (this person means the crossword setter themselves**).
'upset' shows that the letters should be reversed in order.
'and' is a charade indicator (letters next to each other).
'cross' becomes 'x'.
'im' back-to-front is 'mi'.
'mi'+'x'='MIX'
'it's' is the link.
(Other definitions for mix that I've seen before include "= 1,009 (L.)" , "Blend or socialise" , "Amalgamate, blend" , "'Mingle, combine (3)'" , "Get on" .)