Poet expressing pain after desire waned (10)
I believe the answer is:
longfellow
'poet' is the definition.
(Henry Wadsworth Longfellow, 1807-1882)
'expressing pain after desire waned' is the wordplay.
'expressing pain' becomes 'ow' ('ow' is an expression of pain).
'after' is a charade indicator (letters next to each other).
'desire' becomes 'long' (longing is a kind of desiring).
'waned' becomes 'fell' (wane can mean to decrease or fall).
'long'+'fell'='longfell'
'ow' after 'longfell' is 'LONGFELLOW'.
(Other definitions for longfellow that I've seen before include "Hiawatha poet" , "Song of_Hiawatha poet" , "American poet, author of 17" .)