He's a matter of taste (5)
I believe the answer is:
basil
'he's' is the definition.
'a matter of taste' is the wordplay.
'a matter' becomes 'Ba' (Ba is an example).
'of taste' becomes 'sil' (I can't explain this - if you can you should believe this answer much more).
'ba'+'sil'='BASIL'
Can you help me to learn more?
(Other definitions for basil that I've seen before include "Pesto ingredient" , "Herb; man's name" , "Aromatic plant used as a herb" , "Plant; Brush; Fawlty" , "Culinary herb" .)