Somehow able to grasp historical King by way of part of London (9)
I believe the answer is:
belgravia
'part of london' is the definition.
(I've seen this before)
'somehow able to grasp historical king by way' is the wordplay.
'somehow' indicates an anagram.
'to grasp' means one lot of letters goes inside another.
'historical king' becomes 'GR' (short for Georgius Rex, King George in Latin).
'by' is a charade indicator (letters next to each other).
'way' becomes 'via' (I've seen this in other clues).
'able' with letters rearranged gives 'bela'.
'bela' enclosing 'gr' is 'belgra'.
'belgra'+'via'='BELGRAVIA'
'of' acts as a link.
(Other definitions for belgravia that I've seen before include "London locality" , "part of London" , "Area S. of Knightsbridge" , "London area" .)