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