King in regret about left meeting right (5)
I believe the answer is:
ruler
'king' is the definition.
(I've seen this before)
'in regret about left meeting right' is the wordplay.
'in' indicates putting letters inside.
'regret' becomes 'rue' (synonyms).
'about left' becomes 'l' (common abbreviation. I am not sure about the 'about' bit.).
'meeting' is a charade indicator (letters next to each other).
'right' becomes 'r' (common abbreviation).
'rue' enclosing 'l' is 'rule'.
'rule'+'r'='RULER'
Can you help me to learn more?
(Other definitions for ruler that I've seen before include "Measuring strip" , "one responsible for a line?" , "One who holds sway and effects government" , "Head of state" , "Person in power - can draw a line" .)