Then I'm completely confused because gum ammoniac comes from Dorema ammoniacum which is apparently not the answer.
At the moment I have
21D = **A*O*
and you said
21D is a tree(and it's gum)...
and
21D the answer has been locked up(put away)
Sorry, but I just don't understand your clues any more than I understand Picaroon's. But thanks for trying.