Their thing turned out to be justified (2,3,5)
I believe the answer is:
in the right
'be justified' is the definition.
(I've seen this before)
'their thing turned out' is the wordplay.
'turned out' is an anagram indicator.
'their'+'thing'='theirthing'
'theirthing' anagrammed gives 'IN THE RIGHT'.
'to' is the link.
(Other definitions for in the right that I've seen before include "Not wrong" , "Maintaining a justifiable position" , "Conservatives" , "justified" , "Supported by law (or reason)" .)