Experienced about a year's allegiance (6)
I believe the answer is:
fealty
'allegiance' is the definition.
('fealty' can be a synonym of 'allegiance')
'experienced about a year's' is the wordplay.
'experienced' becomes 'felt' (feeling is a kind of experiencing**).
'about' is an insertion indicator.
'year' becomes 'y'.
'felt' placed around 'a' is 'fealt'.
'fealt'+'y'='FEALTY'
(Other definitions for fealty that I've seen before include "Old-fashioned loyalty or allegiance" , "The loyalty that citizens owe their country" , "Vassal's sworn loyalty to a lord (historical)" , "The loyalty that vassals used to swear to their lord" , "Loyalty to feudal superior" .)