Protection for plants? That's old hat! (6)
I believe the answer is:
cloche
'old hat' is the definition.
(old-fashioned type of hat)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for cloche that I've seen before include "Glass cover protecting young plants against cold" , "protection for young stock?" , "Nursery plant protection" , "Seedling cover" , "Protective covering for plants from frost" .)