Crossword Genius

Prepared to fire with very little justification (7-5)

Ross

I believe the answer is:

trigger-happy

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'prepared' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?

I don't understand how the rest of the clue works.

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(Other definitions for trigger-happy that I've seen before include "Quick to use a gun" , "Over-ready to shoot" , "Over-enthusiastic with a gun" , "Irresponsible with firearms" , "inclined to recklessness" .)

I've seen this clue in the Evening Standard.
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