Books about advances restricting to reptiles (9)
I believe the answer is:
tortoises
'reptiles' is the definition.
(tortoise is a kind of reptile)
'books about advances restricting to' is the wordplay.
'books about' becomes 'to' (I can't justify this - if you can you should give a lot more credence to this answer).
'advances' becomes 'rises' ('rise' can be a synonym of 'advance').
'restricting' means one lot of letters goes inside another.
'rises' enclosing 'to' is 'rtoises'.
'to'+'rtoises'='TORTOISES'
Can you help me to learn more?
(Other definitions for tortoises that I've seen before include "Shelled creatures" , "Horny-shelled reptiles" , "The have dome-shaped leathery shells" .)