Gull in trees with aspiration called out (4)
I believe the answer is:
hoax
'gull' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for hoax that I've seen before include "Spoof" , "Leg-pull" , "Fake, such as Piltdown Man" , "Scam" , "Fool" .)