Certain sack is reliable (4,4)
I believe the answer is:
sure fire
'reliable' is the definition.
Both the answer and definition are adjectives.
Perhaps you can see a link between them that I can't see?
'certain sack' is the wordplay.
'certain' becomes 'sure' (synonyms).
'sack' becomes 'fire' (synonyms).
'sure'+'fire'='SURE-FIRE'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for sure fire that I've seen before include "certain to succeed" , "Like bankers" .)