Incompetent bunch? I'm not sure after demonstration (6)
I believe the answer is:
shower
'incompetent bunch?' is the definition.
(shower can mean a group of incompetent people)
'i'm not sure after demonstration' is the wordplay.
'i'm not sure' becomes 'er' (expression of hesitation).
'after' is a charade indicator (letters next to each other).
'demonstration' becomes 'show' (show is a kind of demonstration).
'er' after 'show' is 'SHOWER'.
(Other definitions for shower that I've seen before include "Person uncovering" , "clean" , "Useless people" , "that's revealing?" , "Fall of rain; slack people (slang)" .)