It’s just uncommonly conclusive (4)
I believe the answer is:
only
'it's just' is the definition.
('only' can be a synonym of 'just')
'uncommonly conclusive' is the wordplay.
'ONLY' is hidden in amongst the letters.
However, I'm unsure how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for only that I've seen before include "Single, solitary" , ". . . . the Lonely (sung by Roy Orbison)" , "And nothing more; sole" , "Solely; except that" , "Merely; single" .)