Mad King George some must admit (5)
I believe the answer is:
angry
'mad' is the definition.
(similar in meaning)
'king george some must admit' is the wordplay.
I cannot quite see how this works, but
'king' could be 'gr' (short for Georgius Rex, King George in Latin) and 'gr' is found in the answer.
'some' could be 'any' (similar in meaning) and 'any' is present in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for angry that I've seen before include "Vexed" , "Incensed" , "In a paddy" , "Cross" , "In a temper" .)