Understand old king has a snake (5)
I believe the answer is:
grasp
'understand' is the definition.
(I know that grasp is a kind of the understand)
'old king has a snake' is the wordplay.
'old king' becomes 'GR' (Georgius Rex, Latin for King George).
'has' is a charade indicator (letters next to each other).
'a snake' becomes 'asp' (asp is a kind of snake).
'gr'+'asp'='GRASP'
(Other definitions for grasp that I've seen before include "Support; rugby player" , "Clutch" , "Take firm hold" , "Catch on" , "Seize, hold firmly" .)