How happy gunslingers might be? (7)

I believe the answer is:

trigger

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'be?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for trigger that I've seen before include "make things happen" , "prompt" , "Roy Rogers' horse" , "Gun release mechanism" , "Part of gun" .)

I've seen this clue in The Globe and Mail.

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