Dope, lamebrain, fool -- not all there! (4)
I believe the answer is:
info
'dope' is the definition.
(I've seen this before)
'lamebrain fool not all there' is the wordplay.
'not all there' indicates the answer is hidden within the clue (only some of the letters are to be retained).
'INFO' is hidden in the letters of 'lamebrain fool'.
(Other definitions for info that I've seen before include "Intelligence (abbr)" , "Gen (colloq.)" , "Facts; knowledge gained" , "Gen (abbr)" , "counsel?" .)