In short, it can't fail (4)
I believe the answer is:
cert
'can't fail' is the definition.
I can't tell whether this defines the answer.
'in short it' is the wordplay.
'in' indicates putting letters inside.
'short' becomes 'crt' (I can't explain this - if you can you should believe this answer much more).
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'crt' going around 'e' is 'CERT'.
Can you help me to learn more?
(Other definitions for cert that I've seen before include "Banker" , "Inevitable event" , "Safe bet" , "Infallible bet (slang)" , "One sure to win (slang)" .)