A ruler is in the desk in Greg’s room (4)
I believe the answer is:
king
'a ruler' is the definition.
(I know that king is a type of crowned head)
'in the desk in greg's room' is the wordplay.
'in' indicates the answer is hidden within the clue.
'KING' can be found hidden inside 'desk in gregs room'.
'is' acts as a link.
(Other definitions for king that I've seen before include "eg Wenceslas" , "and 21 Across: Shakespeare play" , "See 22" , "indispensable on board" , "Play set in medieval Britain" .)