Out-and-out guarantee leader’s been ousted (6)
I believe the answer is:
arrant
'out-and-out' is the definition.
(synonyms)
'guarantee leader's been ousted' is the wordplay.
I cannot quite see how this works, but
'been' could be 'ran' (running is a kind of being) and 'ran' is found in the answer.
The remaining letters 'art' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for arrant that I've seen before include "'Complete, stark (6)'" , "Utter and extreme, maybe nonsense" , "Out-and-out, utter" , "Downright - notorious" , "Blatant" .)