Stratagem sure to fail (4)
I believe the answer is:
ruse
'stratagem' is the definition.
(I've seen this before)
'sure to fail' is the wordplay.
'to fail' indicates an anagram (letters go wrong or fail).
'sure' anagrammed gives 'RUSE'.
(Other definitions for ruse that I've seen before include "Sure to be a plot" , "Cunning plan" , "Trick, stratagem" , "Ploy, artifice" , "Cunning scheme" .)