Observes broken stone (5)
I believe the answer is:
notes
'observes' is the definition.
('note' can be a synonym of 'observe')
'broken stone' is the wordplay.
'broken' indicates an anagram.
'stone' with letters rearranged gives 'NOTES'.
(Other definitions for notes that I've seen before include "registers?" , "Doh, re, mi, ..." , "Takes into account" , "Observes; short comments" , "Written comments" .)