Not competent (5)
I believe the answer is:
inept
'not competent' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for inept that I've seen before include "Incompetent or gauche" , "Not competent" , "Absurd or unskilful" , "Showing a lack of competence" , "Blundering" .)