Fidelity, say, within marriage? (10)
I believe the answer is:
allegiance
'fidelity' is the definition.
(allegiance is a kind of fidelity)
'say within marriage' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'within' means one lot of letters goes inside another.
'marriage' becomes 'alliance' (I've seen this before).
'eg' placed inside 'alliance' is 'ALLEGIANCE'.
(Other definitions for allegiance that I've seen before include "commitment to a cause" , "Loyalty to a group or cause" , "duty" , "Loyalty (to a superior)" , "Fidelity" .)