Jack set out play (4)
I believe the answer is:
jest
'play' is the definition.
(thesaurus)
'jack set out' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'set' with letters rearranged gives 'est'.
'j'+'est'='JEST'
(Other definitions for jest that I've seen before include "fun" , "` `Many a true word is spoken in . . . .' '" , "Scoff; playfulness" , "Sally" , "it's not serious, so to speak" .)