Break down code some altered penning page (9)
I believe the answer is:
decompose
'break down' is the definition.
(thesaurus)
'code some altered penning page' is the wordplay.
'altered' indicates anagramming the letters.
'penning' is an insertion indicator.
'page' becomes 'p'.
'code'+'some'='codesome'
'codesome' is an anagram of 'decomose'.
'decomose' enclosing 'p' is 'DECOMPOSE'.
(Other definitions for decompose that I've seen before include "Crumble" , "as rotters do!" , "Decay, disintegrate" , "Go off" , "Spoil" .)