Foolish patient forgetting a time for treatment (5)
I believe the answer is:
inept
'foolish' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for inept that I've seen before include "Hopeless; bungling" , "Clumsy and incompetent" , "Clumsy, bumbling" , "Showing a lack of competence" , "Awkward, showing no skill" .)