Plant found in back parts of Thanet Castle (6)
I believe the answer is:
nettle
'plant' is the definition.
(I know that nettle is a type of weed)
'in back parts of thanet castle' is the wordplay.
'back' says to take the final letters.
'parts of thanet castle' becomes 'ettle' (I can't justify this - if you can you should believe this answer much more).
The last letter of 'in' is 'n'.
'n'+'ettle'='NETTLE'
'found' is the link.
Can you help me to learn more?
(Other definitions for nettle that I've seen before include "Sting or annoy with the plant" , "Provoke" , "Plant to irritate" , "Irritate, annoy" , "Annoy; common weed" .)