Distinctive phrase fool finally changes (5)
I believe the answer is:
idiom
'distinctive phrase' is the definition.
(an idiom is a phrase or saying)
I can't explain the rest of the clue.
(Other definitions for idiom that I've seen before include "Group of words in which the meaning may not be obvious" , "Eg, over the moon" , "'Break a leg' or 'miss the boat'" , "Manner of speaking or writing" , "Characteristic style of a period" .)