Thus stitch regularly disappears (3)
I believe the answer is:
sic
'thus' is the definition.
('sic' can be a synonym of 'thus')
'stitch regularly disappears' is the wordplay.
'regularly disappears' indicates alternate letters (letters removed at regular intervals).
The alternate letters of 'stitch' are 'SIC'.
(Other definitions for sic that I've seen before include "Like" , "Intentionally so written" , "Written exactly thus" , "As it stands, in printing" , "Printing term to denote accuracy though it appears otherwise" .)