Loyalty, say, within marriage (10)
I believe the answer is:
allegiance
'loyalty' is the definition.
(allegiance is a kind of loyalty)
'say within marriage' is the wordplay.
'say' becomes 'eg'.
'within' is an insertion indicator.
'marriage' becomes 'alliance' (I've seen this before).
'eg' put inside 'alliance' is 'ALLEGIANCE'.
(Other definitions for allegiance that I've seen before include "Loyalty to a group or cause" , "commitment to a cause" , "duty" , "Loyalty (to a superior)" , "Fidelity" .)