Tell positively, when certain (6)
I believe the answer is:
assure
'tell positively' is the definition.
(to assure is to tell someone something positively)
'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 "Give confidence to" , "Convince, guarantee" , "Guarantee, promise" , "Cover by life insurance" , "Convince, promise" .)