A part of unusually rich poem (5)
I believe the answer is:
lyric
'poem' is the definition.
(I know that lyric is a type of poem)
'a part of unusually rich' is the wordplay.
'a part of' indicates a hidden word.
'LYRIC' is hidden within 'unusually rich'.
(Other definitions for lyric that I've seen before include "Text of a popular song" , "Cyril (anag.)" , "in musical" , "Short poem of songlike quality" , "Of poetry that expresses thoughts and feelings" .)