Torment poor Fergie endlessly (5)
I believe the answer is:
grief
'torment' is the definition.
(thesaurus)
'poor fergie endlessly' is the wordplay.
'poor' is an anagram indicator.
'endlessly' means to remove the last letter.
'fergie' with its final letter taken off is 'fergi'.
'fergi' anagrammed gives 'GRIEF'.
(Other definitions for grief that I've seen before include "Great mourning" , "Distress" , "Suffering" , "Heartbreak" , "Sharp sorrow" .)