Up to it (4)
I believe the answer is:
'up to it' is the definition.
(up to it means able to do something)
This is the entire clue.
(Other definitions for able that I've seen before include "Competent as, say, sailor" , "Competent - sort of seaman" , "As capable as a mariner" , "Proficient; skilful" , "Fitted" .)