Slapdash DIY nut is messy (6)
I believe the answer is:
untidy
'slapdash' is the definition.
(thesaurus)
'diy nut is messy' is the wordplay.
'is messy' indicates anagramming the letters.
'diy'+'nut'='diynut'
'diynut' with letters rearranged gives 'UNTIDY'.
(Other definitions for untidy that I've seen before include "Dishevelled, messy" , "Higgledy-piggledy" , "Disorderly, messy" , "Slovenly, unkempt" , "Disorderly, dishevelled" .)