One thing's changed understanding (7)
I believe the answer is:
insight
'understanding' is the definition.
(I know that insight is a type of understanding)
'one thing's changed' is the wordplay.
'one' becomes 'i' (Roman numeral).
'changed' is an anagram indicator.
'things' anagrammed gives 'nsight'.
'i'+'nsight'='INSIGHT'
(Other definitions for insight that I've seen before include "Grasp" , "Ability to understand" , "Intuition" , "Acute understanding" , "Special knowledge" .)