Support for King 'enry is following Yorkist leader invading most of capital (8)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

‘AL (Cockney pronunciation of HAL = nickname for King Henry) + IS (from clue text), following the leading letter of Y [orkist], all inserted into ROM [e] (capital city) with the last letter dropped (most of . . .).