Up to a task (4)
I believe the answer is:
able
'up to a task' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for able that I've seen before include "Competent and equipped" , "Of high proficiency" , "Competent as, say, sailor" , "Qualified - expert" , "Competent and fit, like sailor" .)