Agrees to change gun ammunition (6)
I believe the answer is:
grease
'gun ammunition' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'agrees to change' is the wordplay.
'to change' indicates anagramming the letters.
'agrees' is an anagram of 'GREASE'.
Can you help me to learn more?
(Other definitions for grease that I've seen before include "Lubricating agent" , "Musical starring John Travolta and Olivia Newton-John" , "Fat, film" , "Musical that sounds like a country" , "Film with Travolta and Newton-John" .)