Guarantee Head avoids criticism (6)
I believe the answer is:
ensure
'guarantee' is the definition.
(I know that guarantee can be written as ensure)
'head avoids criticism' is the wordplay.
'head avoids' means to remove the first letter.
'criticism' becomes 'censure' (censure means criticism or rebuke).
'censure' with its initial letter removed is 'ENSURE'.
(Other definitions for ensure that I've seen before include "Make certain, secure" , "Make certain (something will occur)" , "Make certain or safe" , "Make certain, clinch" , "See run (anag)" .)