Any messing about is negative (3)
I believe the answer is:
nay
'is negative' is the definition.
(I've seen this before)
'any messing about' is the wordplay.
'messing about' indicates an anagram.
'any' anagrammed gives 'NAY'.
(Other definitions for nay that I've seen before include "In point of fact - a parliamentary vote against" , "disagreement" , "Archaic negative" , "Denial" , "No (dialect); or rather" .)