One giant, messy denial (8)
I believe the answer is:
negation
'denial' is the definition.
(negation is a kind of denial)
'one giant messy' is the wordplay.
'messy' is an anagram indicator.
'one'+'giant'='onegiant'
'onegiant' is an anagram of 'NEGATION'.
(Other definitions for negation that I've seen before include "Denial of something's existence" , "Disaffirmation" , "Cancellation" , "Refutation by Antigone" , "people" .)