Old character is prickly sort (5)
I believe the answer is:
thorn
This clue is a double definition.
'old character' is the first definition.
(letter used in Old English)
'prickly sort' is the second definition.
(prickly part of a plant)
'is' acts as a link.
(Other definitions for thorn that I've seen before include "Barb" , "Pointed growth" , "Pointed extension of leaf" , "Sharp point on plant" , "Sharp-pointed tip on a stem" .)