Understood attic is messy (5)
I believe the answer is:
tacit
'understood' is the definition.
(I know that understood can be written as tacit)
'attic is messy' is the wordplay.
'is messy' indicates anagramming the letters.
'attic' anagrammed gives 'TACIT'.
(Other definitions for tacit that I've seen before include "Implicitly understood" , "Wordless" , "Silent - understood" , "Implied, not spoken" , "Attic? (anag)" .)