Flower book (6)
I believe the answer is:
annual
This could be a double definition.
'flower' is the first definition.
The definition and answer can be both plants as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'book' is the second definition.
(annual is a kind of book)
Can you help me to learn more?
(Other definitions for annual that I've seen before include "Occurring once every year" , "Plant that lives and dies within one year" , "Occurring yearly" , "Hardy it may be" , "Yearly publication" .)