Be against entering into falsehood (4)
I believe the answer is:
live
'be' is the definition.
(living is a kind of being)
'against entering into falsehood' is the wordplay.
'against' becomes 'v' (abbreviation for 'versus').
'entering into' indicates putting letters inside.
'falsehood' becomes 'lie' (lie is a kind of falsehood).
'v' inserted within 'lie' is 'LIVE'.
(Other definitions for live that I've seen before include "Exist; not recorded" , "Be - unrecorded" , "Alert and enthusiastic person" , "Be - positive" , "Survive" .)