Guarantee when certain (6)
I believe the answer is:
assure
'guarantee' is the definition.
(I know that guarantee can be written as assure)
'when certain' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'certain' becomes 'sure' (synonyms).
'as'+'sure'='ASSURE'
(Other definitions for assure that I've seen before include "Declare confidently, promise" , "Make sure, guarantee" , "Cover with a policy" , "Impart confidence" , "Make (something) certain to happen" .)