Thick slice of bread I put safely back (6)
I believe the answer is:
stupid
'thick' is the definition.
(similar in meaning)
'slice of bread i put safely back' is the wordplay.
'slice of' says the answer is hidden in the clue.
'back' says the letters should be written backwards.
'diputs' is hidden within 'bread i put safely'.
'diputs' written backwards gives 'STUPID'.
(Other definitions for stupid that I've seen before include "Imbecilic" , "Daft" , "Obtuse" , "Ass" , "Not intelligent, foolish" .)