Ground rent a mess, a mess! (5-4)
I believe the answer is:
mares-nest
'mess' is the definition.
(mare's-nest can mean a confused or untidy place)
'ground rent a mess a' is the wordplay.
'ground' indicates an anagram ('grind' the letters into a new form).
'rent'+'mess'+'a'='rentmessa'
'rentmessa' anagrammed gives 'MARES-NEST'.
(Other definitions for mares-nest that I've seen before include "Disordered situation" , "Steersman found in mess or confused condition" , "Muddle; illusory discovery" .)