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 subsidiary indication.
'with' is a charade indicator (letters next to each other).
'key' becomes 'f' (I've seen this in other clues).
'one's' becomes 'is' ('I' can be a synonym of 'one').
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'f' put next to 'is' is 'fis'.
'to get' is the link.
(Other definitions for fissure that I've seen before include "Long, narrow opening", "Split, crack", "Split - cleavage", "Cleft", "Crack in rock", "Long, narrow crack" and "Crevice".)