Give confidence when certain (6)
I believe the answer is:
assure
'give confidence' is the definition.
(I've seen this before)
'when certain' is the wordplay.
'when' becomes 'as'.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'as'+'sure'='ASSURE'
(Other definitions for assure that I've seen before include "State positively and confidently, guarantee" , "Make (something) certain to happen" , "Impart confidence" , "Make sure, guarantee" , "Cover by life insurance" .)