Bonehead in IT withholds screens (6)
I believe the answer is:
nitwit
'bonehead' is the definition.
(I've seen this in another clue)
'in it withholds screens' is the wordplay.
'screens' indicates the answer is hidden within the clue.
'NITWIT' is hidden in the letters of 'in it withholds'.
(Other definitions for nitwit that I've seen before include "Silly fool" , "Foolish person like humorous louse" , "Blockhead" , "Foolish person like funny louse?" , "'Foolish, silly person (6)'" .)