Ruler, to some extent, back in geometry (4)
I believe the answer is:
king
'ruler' is the definition.
(king is a kind of ruler)
'to some extent back in geometry' is the wordplay.
'to some extent' says the answer is hidden in the clue (I've seen 'extent of' mean this (I've seen this in other clues)).
'KING' is hidden in the letters of 'back in geometry'.
(Other definitions for king that I've seen before include "US civil rights leader, assassinated 1968" , "Playing-card" , "Sovereign" , "Court card" , "Man with a crown" .)