Not true fleas (5)
I believe the answer is:
false
'not true' is the definition.
(I know this)
'fleas' is the wordplay.
'fleas' can be anagrammed to 'FALSE'.
But, I'm not sure how the anagram is indicated.
Can you help me to learn more?
(Other definitions for false that I've seen before include "Invalid; untrue" , "Forged" , "Wrong" , "Bogus" , "Artificial" .)