With key one's certain to get an opening (7)
I believe the answer is:
Here is my best explanation:
'an opening' is the definition.
(fissure is a kind of opening)
'with key one's certain' is the wordplay.
'with' says to put letters next to each other.
'key' becomes 'f' (I've seen this in other clues**).
'one's' becomes 'is' (roman numeral).
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'f' next to 'is' is 'fis'.
'to get' acts as a link.
(Other definitions for fissure that I've seen before include "Long, narrow opening", "Crack in rock", "Split, crack", "Cleft", "Long, narrow crack", "Split - cleavage", "Crevice" and "Deep crack in rock".)