One who regrets accepting pound or sovereign? (5)
I believe the answer is:
ruler
'sovereign?' is the definition.
(sovereign is a kind of ruler)
'one who regrets accepting pound' is the wordplay.
'one who regrets' becomes 'ruer' (I've seen this in another clue).
'accepting' indicates putting letters inside (inserted letters are accepted).
'pound' becomes 'L' (abbreviation e.g. Lsd - pounds, shillings, pence).
'ruer' enclosing 'l' is 'RULER'.
'or' acts as a link.
(Other definitions for ruler that I've seen before include "Controller" , "measure applied in school?" , "that should go straight" , "One for drawing lines" , "one responsible for a line?" .)