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?' indicates an anagram (I've seen 'flying' mean this (letters fly around into new positions)).
'fleas' with letters rearranged gives 'FALSE'.
(Other definitions for false that I've seen before include "Not genuine or real" , "Bogus" , "Sham" , "Spurious, untrue" , "Untrue - treacherous - counterfeit" .)