Plants found in reference books (7)
I believe the answer is:
annuals
This clue could be a double definition.
'plants found in' is the first definition.
'annuals' can be an answer for 'plants' (I've seen this before). I am unsure of the 'found in' bit.
'reference books' is the second definition.
(annual is a kind of reference book)
Can you help me to learn more?
(Other definitions for annuals that I've seen before include "Yearly books" , "Yearly plants" , "regular books" , "Yearly publications" , "Books published once a year" .)