Utterly nasty tribal leaders regret being false (6)
I believe the answer is:
untrue
'being false' is the definition.
('untrue' can be similar in meaning to 'false')
'utterly nasty tribal leaders regret' is the wordplay.
'leaders' suggests taking the first letters.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
The initial letters of 'utterly nasty tribal' is 'unt'.
'unt'+'rue'='UNTRUE'
(Other definitions for untrue that I've seen before include "Not in accordance with fact" , "Sleeping around" , "Wrong" , "like the magnetic north?" , "Not loyal" .)