Fleas fly? Not true (5)
I believe the answer is:
false
'not true' is the definition.
('false' is the opposite to 'true')
'fleas fly?' is the wordplay.
'fly?' is an anagram indicator (I've seen 'flying' mean this (letters fly around into new positions)).
'fleas' is an anagram of 'FALSE'.
(Other definitions for false that I've seen before include "Not genuine or real" , "Bogus" , "Sham" , "Spurious, untrue" , "Untrue - treacherous - counterfeit" .)