Strangely true-hearted when unwell (5,3,7)
I believe the answer is:
under the weather
'unwell' is the definition.
(thesaurus)
'strangely true-hearted when' is the wordplay.
'strangely' indicates an anagram.
'true'+'hearted'+'when'='trueheartedwhen'
'trueheartedwhen' is an anagram of 'UNDER THE WEATHER'.
(Other definitions for under the weather that I've seen before include "Sickening" , "Sick" , "not so good" , "Indisposed" , "put umbrellas up here?" .)