Fool's house with a cross (4)
I believe the answer is:
hoax
'fool's' is the definition.
(thesaurus)
'house with a cross' is the wordplay.
'house' becomes 'ho' (abbreviation for house).
'with' says to put letters next to each other.
'cross' becomes 'x'.
'ho'+'a'+'x'='HOAX'
(Other definitions for hoax that I've seen before include "Humorous or malicious deception" , "A plan to deceive" , "Prank" , "A fraud or deception" , "Fake, such as Piltdown Man" .)