Critical comrade impounding four books (7)
I believe the answer is:
pivotal
'critical' is the definition.
(I've seen this before)
'comrade impounding four books' is the wordplay.
'comrade' becomes 'pal' (I've seen this in another clue).
'impounding' is an insertion indicator.
'four' becomes 'IV' (Roman numerals).
'books' becomes 'OT' (Old Testament).
'iv'+'ot'='ivot'
'pal' placed around 'ivot' is 'PIVOTAL'.
(Other definitions for pivotal that I've seen before include "Crucial, critical" , "Of central importance" , "Crucially important" , "Central - decisive" , "Of crucial importance" .)