Degenerate dug nude upon books’ covers (2,2,3)
I believe the answer is:
go to pot
'degenerate' is the definition.
(thesaurus)
'dug nude upon books' covers' is the wordplay.
'dug' becomes 'got' (I've seen this in another clue).
'nude' says to take the centre (the word is stripped of its outer letters).
'books'' becomes 'OT' (Old Testament).
'covers' means one lot of letters goes inside another.
The centre of 'upon' is 'po'.
'po' put into 'ot' is 'opot'.
'got'+'opot'='GO TO POT'
(Other definitions for go to pot that I've seen before include "If you want a cup of tea" , "Fall into neglect" , "Deteriorate completely - ready to be cooked?" , "run down" , "Tea bags will" .)