Loyalty, say, found in union (10)
I believe the answer is:
allegiance
'loyalty' is the definition.
(allegiance is a kind of loyalty)
'say found in union' is the wordplay.
'say' becomes 'eg'.
'found in' indicates putting letters inside.
'union' becomes 'alliance' (alliance is a kind of union**).
'eg' going inside 'alliance' is 'ALLEGIANCE'.
(Other definitions for allegiance that I've seen before include "duty" , "Fidelity" , "Loyalty to a group or cause" , "Loyalty (to a superior)" , "commitment to a cause" .)