They contain seeds one buries, as mentioned (7)
I believe the answer is:
berries
'they' is the definition.
Both the definition and answer are plural nouns.
Perhaps you can see a link between them that I can't see?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for berries that I've seen before include "They can be black, blue or straw" , "Beer, sir? (anag.)" .)