Loudest one interrupts the most inquisitive (8)
I believe the answer is:
noisiest
'loudest' is the definition.
('noisy' is associated in meaning with 'loud')
'one interrupts the most inquisitive' is the wordplay.
'one' becomes 'i' (Roman numeral).
'interrupts' is an insertion indicator.
'the most inquisitive' becomes 'nosiest' (I've seen this before).
'i' going within 'nosiest' is 'NOISIEST'.
(Other definitions for noisiest that I've seen before include "Biggest racket" , "Making the loudest sounds" , "party being at this extreme?" , "Making the most loud sounds" , "Making the most sound" .)