Not pleased to submit to old killer (8)
I believe the answer is:
crossbow
'old killer' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'not pleased to submit' is the wordplay.
'not pleased' becomes 'cross' (I've seen this in another clue).
'to submit' becomes 'bow' ('bow' can be a synonym of 'submit').
'cross'+'bow'='CROSSBOW'
'to' is the link.
Can you help me to learn more?
(Other definitions for crossbow that I've seen before include "historic weapon" , "can precipitate a quarrel" , "William Tell's weapon" , "it could be wounding" , "Weapon for shooting bolts" .)