Page, torn, will hang open (4)
I believe the answer is:
gape
'hang open' is the definition.
(I've seen this in another clue)
'page torn' is the wordplay.
'torn' is an anagram indicator.
'page' anagrammed gives 'GAPE'.
'will' acts as a link.
(Other definitions for gape that I've seen before include "Failure to meet" , "Look at with wide-open mouth" , "Look with mouth wide open" , "Stare stupidly" , "Stare with open mouth in wonder" .)