Being faithful, such as in marriage (10)
I believe the answer is:
allegiance
'being faithful' is the definition.
'as in marriage' is the wordplay.
'as' becomes 'eg' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'marriage' becomes 'alliance' (I've seen this before).
'eg' placed inside 'alliance' is 'ALLEGIANCE'.
'such' is the link.
(Other definitions for allegiance that I've seen before include "Fidelity" , "commitment to a cause" , "duty" , "Loyalty to a group or cause" , "Loyalty (to a superior)" .)