Sentence:  He plays with the late Watts-Dunton and gets good fun out of it; he rips open the sham, <WSD>unstays</WSD> the imperfect, sonnet ; he tackles all fretters of the great English instrument and worries them all, yet with a fine sanity. 
Meaning of the word marked with <WSD> tags:  To undo the establishment of; disestablish; dissolve; divest.