Almost immediately arrests one without stress (6)
I believe the answer is:
atonic
'without stress' is the definition.
(term from linguistics)
'almost immediately arrests one' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'immediately' becomes 'atonce' (I've seen this before).
'arrests' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'atonce' with its final letter taken away is 'atonc'.
'atonc' going around 'i' is 'ATONIC'.
(Other definitions for atonic that I've seen before include "Unstressed (poetry)" , "(In poetry) unaccented" .)