DIY nut terribly messy (6)
I believe the answer is:
untidy
'messy' is the definition.
(synonyms)
'diy nut terribly' is the wordplay.
'terribly' indicates an anagram.
'diy'+'nut'='diynut'
'diynut' is an anagram of 'UNTIDY'.
(Other definitions for untidy that I've seen before include "Messy, not neat" , "Slovenly, unkempt" , "Not neatly arranged" , "Not inclined to be neat" , "in a state" .)