Poem's code is nonsensical rot! (9)
I believe the answer is:
decompose
'rot' is the definition.
(synonyms)
'poem's code is nonsensical' is the wordplay.
'is nonsensical' indicates anagramming the letters.
'poems'+'code'='poemscode'
'poemscode' is an anagram of 'DECOMPOSE'.
(Other definitions for decompose that I've seen before include "Crumble" , "Rot" , "Scooped me in order to decay" , "Go off" , "Spoil" .)