Prick (5)
(Other definitions for thorn that I've seen before include "Pointed growth" , "tip of R [ROSE]" , "Something 20 [SHARP]" , "North (anag) -- point" , "eg Prickle on rose" .)
'prick' is the definition.
(I know that prick is a more specific form of the action prick)
This is the entire clue.
(Other definitions for sting that I've seen before include "Smart - cheat" , "(Bee) wound" , "piece of deception" , "18, sharp pain" , "fleece?" .)