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' means one lot of letters goes inside another.
'partnership' becomes 'alliance' (I've seen this before).
'eg' going 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" , "Fidelity" , "Loyalty (to a superior)" .)