Managed to penetrate English poet's creative language (9)
I believe the answer is:
esperanto
'managed to penetrate english poet's creative language' is the definition.
I can't tell whether this defines the answer.
'to penetrate english poet's creative language' is the wordplay.
'to penetrate' indicates putting letters inside.
'english' becomes 'e' (abbreviation).
'creative' is an anagram indicator.
'language' becomes 'ran' (I am not sure about this - if you are sure you should believe this answer much more).
'e'+'poets'='epoets'
'epoets' is an anagram of 'espeto'.
'espeto' going around 'ran' is 'ESPERANTO'.
(Other definitions for esperanto that I've seen before include "artificial tongue" , "Parse note in the artificial language" , "Universal language" , "Personate in artificial language" , "contrived speech" .)