Is __ to; can
I believe the answer is:
able
'is to can' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for able that I've seen before include "Of high proficiency" , "... was I ere I saw Elba (palindrome)" , "Competent - sort of seaman" , "Having great proficiency or intelligence" , "Competent as, say, sailor" .)