Books developed panic mode (9)
I believe the answer is:
compendia
'books' is the definition.
(I've seen this before)
'developed panic mode' is the wordplay.
'developed' indicates anagramming the letters.
'panic'+'mode'='panicmode'
'panicmode' is an anagram of 'COMPENDIA'.
(Other definitions for compendia that I've seen before include "summaries of work" , "set of encyclopaedias" , "books" , "Lots of puzzles" , "Dome panic (anag) - summaries or collections" .)