Ruler found in desk in gallery (4)
I believe the answer is:
king
'ruler' is the definition.
(I know that king is a type of ruler)
'in desk in gallery' is the wordplay.
'in' indicates the answer is hidden within the clue.
'KING' is hidden within 'desk in gallery'.
'found' is the link.
(Other definitions for king that I've seen before include "Martin Luther -" , "One on throne" , "See 22" , "In Christianity, God or Jesus Christ" , "Playing-card" .)