Untruth some believe (3)
I believe the answer is:
lie
'untruth' is the definition.
(I've seen this before)
'some believe' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'LIE' is hidden in the letters of 'believe'.
(Other definitions for lie that I've seen before include "Tell untruths" , "create fiction" , "habitat for animal or bird" , "Stay hidden" , "Pork pie" .)