Fidelity, extremely endearing in marriage (10)
I believe the answer is:
allegiance
'fidelity' is the definition.
(allegiance is a kind of fidelity)
'extremely endearing in marriage' is the wordplay.
'extremely' means to remove the middle letters.
'in' means one lot of letters goes inside another.
'marriage' becomes 'alliance' (I've seen this before).
'endearing' with its centre taken out is 'eg'.
'eg' inserted into 'alliance' is 'ALLEGIANCE'.
(Other definitions for allegiance that I've seen before include "duty" , "Loyalty (to a superior)" , "Loyalty to a group or cause" , "Fidelity" , "commitment to a cause" .)