Glitz, a row in supposition (9)
I believe the answer is:
gaudiness
'glitz' is the definition.
(synonyms)
'a row in supposition' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
'row' could be 'din' (both can mean a loud disturbance) and 'din' is located in the answer.
'supposition' could be 'guess' (synonyms) and 'guess' is located in the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for gaudiness that I've seen before include "Ostentation" , "Tasteless showiness" .)