It's not true about fleas (5)
I believe the answer is:
false
'it's not true' is the definition.
(I know this)
'about fleas' is the wordplay.
'about' indicates anagramming the letters.
'fleas' is an anagram of 'FALSE'.
(Other definitions for false that I've seen before include "Incorrect" , "Forged" , "Artificial" , "Misleading" , "Not in accordance with fact" .)