Poem composed by Cyril (5)
I believe the answer is:
lyric
'poem' is the definition.
(lyric is a kind of poem)
'composed by cyril' is the wordplay.
'composed by' is an anagram indicator (I've seen this in other clues).
'cyril' anagrammed gives 'LYRIC'.
(Other definitions for lyric that I've seen before include "Expressing poetical emotions" , "A song or poem" , "Poem - verse - London theatre" , "having a light quality" , "(Of poetry) expressing emotion" .)