Poet no end upset! (5)
I believe the answer is:
donne
'poet' is the definition.
(John Donne, 1572-1631)
'no end upset' is the wordplay.
'upset' is an anagram indicator.
'no'+'end'='noend'
'noend' is an anagram of 'DONNE'.
(Other definitions for donne that I've seen before include "basic assumption" , "English preacher and metaphysical poet, d. 1631" , "John -, English poet" , "Writer of poetry" .)