Guarantee heading off reprimand (6)
I believe the answer is:
ensure
'guarantee' is the definition.
('ensure' can be a synonym of 'guarantee')
'heading off reprimand' is the wordplay.
'heading off' means to remove the first letter.
'reprimand' becomes 'censure' ('censure' can be a synonym of 'reprimand').
'censure' with its first letter taken off is 'ENSURE'.
(Other definitions for ensure that I've seen before include "Make certain or safe" , "Protect" , "warrant" , "Guarantee" , "Make certain, secure" .)