Eccentric retains key allegiance (6)
I believe the answer is:
fealty
'allegiance' is the definition.
(I know that allegiance can be written as fealty)
'eccentric retains key' is the wordplay.
'eccentric' becomes 'fey' (I've seen this in other clues).
'retains' means one lot of letters goes inside another.
'key' becomes 'alt' (the Alt keyboard button).
'fey' enclosing 'alt' is 'FEALTY'.
(Other definitions for fealty that I've seen before include "Loyalty or allegiance as once to lord" , "Loyalty to feudal superior" , "The loyalty that vassals used to swear to their lord" , "Old-fashioned loyalty or allegiance" , "The loyalty that citizens owe their country" .)