Wilt it tag unfinished keystone? (7)

I believe the answer is:

sagitta

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'keystone?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'wilt it tag unfinished' is the wordplay.
'wilt' becomes 'sag' (I've seen this in another clue).
'unfinished' means to remove the last letter.
'tag' with its last letter taken off is 'ta'.
'sag'+'it'+'ta'='SAGITTA'

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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