Leave from son in monarchy (6)
I believe the answer is:
resign
'leave' is the definition.
(thesaurus)
'son in monarchy' is the wordplay.
'son' becomes 's'.
'in' is an insertion indicator.
'monarchy' becomes 'reign' (I've seen this before).
's' going inside 'reign' is 'RESIGN'.
'from' acts as a link.
(Other definitions for resign that I've seen before include "Turn over" , "Step down" , "Quit" , "Hand in one's notice" , "Give up office" .)