It can't fail to become whole (6)
I believe the answer is:
intact
'whole' is the definition.
(similar in meaning)
'it can't fail' is the wordplay.
'fail' indicates anagramming the letters (letters go wrong or fail).
'it'+'cant'='itcant'
'itcant' anagrammed gives 'INTACT'.
'to become' is the link.
(Other definitions for intact that I've seen before include "Entire and complete" , "Complete - unbroken" , "Whole and undamaged" , "Whole and unharmed" , "Complete - undamaged" .)