Equally certain, can throw in a guarantee (9)
I believe the answer is:
assurance
'a guarantee' is the definition.
(I've seen this before)
'equally certain can throw in' is the wordplay.
'equally' becomes 'as' ('as' can be a synonym of 'equally').
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'throw' indicates an anagram.
'in' means one lot of letters goes inside another.
'can' with letters rearranged gives 'anc'.
'sure' placed around 'anc' is 'surance'.
'as'+'surance'='ASSURANCE'
(Other definitions for assurance that I've seen before include "Guarantee - belief in oneself" , "(Life) cover" , "Confidence, certainty" , "Confidence, composure" , "Certainty; life insurance" .)