Loyalty, for example, in union (10)
I believe the answer is:
'loyalty' is the definition.
(allegiance is a kind of loyalty)
'example in union' is the wordplay.
'example' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'in' is an insertion indicator.
'union' becomes 'alliance' (alliance is a kind of union).
'eg' inserted inside 'alliance' is 'ALLEGIANCE'.
'for' acts as a link.
(Other definitions for allegiance that I've seen before include "duty" , "commitment to a cause" , "Loyalty to a group or cause" , "Loyalty (to a superior)" , "Fidelity" .)