Fit for duty (4)
I believe the answer is:
able
'fit for duty' 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 "Having great proficiency or intelligence" , "Practised" , "Adept" , "... was I ere I saw Elba (palindrome)" , "Having the ability to do something" .)