Silly deputy head at the back (4)
I believe the answer is:
daft
'silly' is the definition.
(I've seen this in another clue)
'deputy head at the back' is the wordplay.
'head' suggests taking the first letters.
'at' means one lot of letters go next to another.
'the back' becomes 'aft' (at the back of a ship).
The first letter of 'deputy' is 'd'.
'd'+'aft'='DAFT'
(Other definitions for daft that I've seen before include "Bats!" , "Potty" , "Half-witted" , "Barmy, crackers" , "Informally silly or foolish" .)