...guarantee principal is free from blame (6)
I believe the answer is:
ensure
'guarantee' is the definition.
('ensure' can be a synonym of 'guarantee')
'principal is free from blame' is the wordplay.
'principal is free from' means to remove the first letter (the first or principal letter is removed).
'blame' becomes 'censure' ('censure' can be a synonym of 'blame').
'censure' with its initial letter taken away is 'ENSURE'.
(Other definitions for ensure that I've seen before include "Make certain, clinch" , "Make certain, secure" , "Make certain, confirm" , "warrant" , "Make certain that (something occurs)" .)