A desert is in a mess (5)
I believe the answer is:
sinai
'a desert' is the definition.
(Sinai is an example)
'is in a mess' is the wordplay.
'mess' indicates an anagram.
'is'+'in'+'a'='isina'
'isina' is an anagram of 'SINAI'.
(Other definitions for sinai that I've seen before include "Mount on which Moses received the Ten Commandments" , "Mountainous Egyptian peninsula" , "Deserted area" , "Egyptian location" , "Law handed down here" .)