Old guards about to get settled (6)
I believe the answer is:
Here is my best explanation:
'settled' is the definition.
'old guards about' is the subsidiary indication.
'old' becomes 'aged' ('aged' can be similar in meaning to 'old').
'guards' means one lot of letters goes inside another.
'about' becomes 're' (common abbreviation for 'with reference to'**).
'aged' going around 're' is 'agreed'.
'to get' is the link.
(Other definitions for agreed that I've seen before include "ratified", "Accorded", "Approved", "Said yes", "Saw eye to eye", "Corresponded", "Tallied - reconciled", "Was consistent", "granted" and "acceptable".)