Poetry covering page in old books was extravagant (9)
I believe the answer is:
overspent
'was extravagant' is the definition.
(to overspend is to spend extravagantly)
'poetry covering page in old books' is the wordplay.
'poetry' becomes 'vers' (I've seen this before).
'covering page' becomes 'pen' (I am not sure about this - if you are sure you should believe this answer much more).
'in' indicates putting letters inside.
'old books' becomes 'OT' (Old Testament).
'vers'+'pen'='verspen'
'verspen' placed into 'ot' is 'OVERSPENT'.
Can you help me to learn more?
(Other definitions for overspent that I've seen before include "Beyond budget" , "broke" , "Invested too much" , "Bought too much" , "exhausted" .)