Old character that is prickly (5)
I believe the answer is:
thorn
I believe this is a double definition.
'old character' is the first definition.
(letter used in Old English)
'is prickly' is the second definition.
(I've seen this before)
'that' acts as a link.
(Other definitions for thorn that I've seen before include "Sharp point on a shrub" , "Feature of rose tree" , "Prickle on a plant" , "sharp object" , "Sharp woody projection" .)