Again take up study, quietly immersed in books (7)
I believe the answer is:
readopt
'again take up' is the definition.
('adopt' can be a synonym for 'take up' so perhaps 'readopt' is OK for 'again take up')
'study quietly immersed in books' is the wordplay.
'study' becomes 'read' (synonyms).
'quietly' becomes 'p' (p for piano and quiet).
'immersed in' is an insertion indicator.
'books' becomes 'OT' (abbreviation for Old Testament).
'p' put into 'ot' is 'opt'.
'read'+'opt'='READOPT'
(Other definitions for readopt that I've seen before include "choose again" , "select candidate again" .)