Loyalty, say, in partnership (10)
I believe the answer is:
allegiance
'loyalty' is the definition.
(allegiance is a kind of loyalty)
'say in partnership' is the wordplay.
'say' becomes 'eg' (both can mean for example).
'in' is an insertion indicator.
'partnership' becomes 'alliance' (I've seen this before).
'eg' placed into 'alliance' is 'ALLEGIANCE'.
(Other definitions for allegiance that I've seen before include "Loyalty to a group or cause" , "commitment to a cause" , "duty" , "Fidelity" , "Loyalty (to a superior)" .)