It's changed, but not significantly (3)
I believe the answer is:
tis
'not significantly' is the definition.
I can't judge whether this defines the answer.
'it's changed' is the wordplay.
'changed' indicates an anagram.
'its' anagrammed gives 'TIS'.
'but' is the link.
Can you help me to learn more?
(Other definitions for tis that I've seen before include "McCourt work" , "Contraction of it is" , "It is shortened" , "... the season to be jolly" , "For poet, that's" .)