Denial improperly fastened (6)
I believe the answer is:
nailed
'fastened' is the definition.
(I've seen this in another clue)
'denial improperly' is the wordplay.
'improperly' indicates an anagram (I've seen this in other clues).
'denial' is an anagram of 'NAILED'.
(Other definitions for nailed that I've seen before include "Fastened with metal pins" , "Fixed; lead in (anag.)" , "Hammered in" , "Fixed - caught" , "Daniel (anag.)" .)