Final message poet composed with guile — waste of time! (8)
I believe the answer is:
epilogue
'final message' is the definition.
(eg the end of a book)
'poet composed with guile waste of time' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is found within the answer.
an anagram of 'guile' is 'ilgue' which is located in the remaining letters.
This explanation may well be incorrect...
(Other definitions for epilogue that I've seen before include "Final part of book" , "End of story" , "Afterword" , "Concluding section of book, play, etc" , "dramatic speech" .)