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