The sure thing (8,7)
I believe the answer is:
definite article
'the' is the definition.
('the' is the definite article in English)
'sure thing?' is the wordplay.
'sure' becomes 'definite' (synonyms).
'thing?' becomes 'article' (I've seen this in another clue).
'definite'+'article'='DEFINITE ARTICLE'
(Other definitions for definite article that I've seen before include "Black and white feature in paper" , "In English, it's always the" , "part of speech" , "Maybe the" .)