What three endure, treated poorly (5,3,7)
I believe the answer is:
under the weather
'poorly' is the definition.
(thesaurus)
'what three endure treated' is the wordplay.
'treated' is an anagram indicator.
'what'+'three'+'endure'='whatthreeendure'
'whatthreeendure' is an anagram of 'UNDER THE WEATHER'.
(Other definitions for under the weather that I've seen before include "where you have to look for this crossword?" , "not so good" , "Out of sorts" , "Sickening" , "the worse for wear" .)