One who knows everything later changed in taste (5,5)
I believe the answer is:
smart aleck
'one who knows everything' is the definition.
(term for a know-it-all)
'later changed in taste' is the wordplay.
'changed' indicates an anagram.
'in' is an insertion indicator.
'taste' becomes 'smack' (a smack of something is a taste of it).
'later' anagrammed gives 'rtale'.
'rtale' put into 'smack' is 'SMART-ALECK'.
(Other definitions for smart aleck that I've seen before include "Wise guy" , "Upstart" , "Know-all" .)