King sheltered by a knight, aged chap (6)
I believe the answer is:
Here is my best explanation:
'chap' is the definition.
'king sheltered by a knight aged' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'sheltered by' is an insertion indicator.
'knight' becomes 'n' (chess).
'aged' becomes 'old' ('old' can be similar in meaning to 'aged').
'r' inserted inside 'an' is 'arn'.
(Other definitions for arnold that I've seen before include "-- Wesker, playwright", "Matthew --; -- Bennett", "Sir Malcolm -, composer (Bridge over the River Kwai)", "Headmaster, poet or composer", "Matthew --, English poet", "Sir Malcolm -, English composer", "Victorian poet", "headmaster", "Essayist" and "Overcome (obstacle)".)