Stays to endorse a number in some armour (7)
I believe the answer is:
corslet
'some armour' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?
'stays to endorse a number in' is the wordplay.
'stays' becomes 'corset' ('corset' can be a synonym of 'stays').
'to endorse a number' becomes 'l' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'corset' enclosing 'l' is 'CORSLET'.
Can you help me to learn more?
(Another definition for corslet that I've seen is " Piece of armour".)