Certainty; something easy (5)
I believe the answer is:
cinch
This clue is a double definition.
'certainty' is the first definition.
(I've seen this in another clue)
'something easy' is the second definition.
(I've seen this in another clue)
(Other definitions for cinch that I've seen before include "piece of cake" , "A walk in the park" , "In slang, a certainty or easy task" , "simple job" , "Something easily done" .)