Some people believe nobody is on the level (4)
I believe the answer is:
even
'on the level' is the definition.
(I've seen this in another clue)
'some people believe nobody' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'EVEN' can be found hidden inside 'people believe nobody'.
'is' acts as a link.
(Other definitions for even that I've seen before include "Like 2018 but not 2017" , "time when sun goes down for the poet" , "Regular - smooth" , "Not odd; flat" , "of consistent quality" .)