Crossword Genius

After conditional discharge, witness becomes more irritable (7)

Ross

I believe the answer is:

testier

'more irritable' is the definition.
('testy' can be a synonym of 'irritable')

I cannot understand how the remainder of the clue works.

(Other definitions for testier that I've seen before include "more crusty" , "More likely to become annoyed" , "more irritable" .)

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