Their thorns can cause sores (5)
I believe the answer is:
roses
'their thorns' is the definition.
I can't judge whether this definition defines the answer.
'cause sores' is the wordplay.
'sores' can be anagrammed to 'ROSES'.
However, I am not sure how this is indicated.
'can' is the link.
Can you help me to learn more?
(Other definitions for roses that I've seen before include "Flowers for watering cans?" , "Bloomers" , "northern counties, symbolically" , "''. . . . . are red, violets are blue''" , "''They are not long, the days of wine and ...'' (Dowson)" .)