About to turn and believe warrant (8)
I believe the answer is:
accredit
'warrant' is the definition.
(thesaurus)
'about to turn and believe' is the wordplay.
'about' becomes 'ca' (abbreviation for 'circa').
'to turn' says the letters should be written backwards.
'and' is a charade indicator (letters next to each other).
'believe' becomes 'credit' (I've seen this in another clue).
'ca' back-to-front is 'ac'.
'ac'+'credit'='ACCREDIT'
(Other definitions for accredit that I've seen before include "Attribute (something to)" , "Certify, ascribe" , "As envoy recommend" , "Officially recognise" , "Cat cried (anag.)" .)