Rose's sharp part (5)
I believe the answer is:
thorn
'rose's sharp part' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for thorn that I've seen before include "Sharp point on, eg, rose" , "Old Norse character" , "Spiny bush" , "Sharp woody projection" , "It causes irritation and annoyance" .)