Disbelief at fitting in puzzle pieces (9)
I believe the answer is:
amazement
'disbelief' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'at fitting in puzzle pieces' is the wordplay.
'at' is a charade indicator (letters next to each other).
'fitting in' becomes 't' (I can't justify this - if you can you should believe this answer much more).
'puzzle' becomes 'amaze' (both can mean to astound or confuse).
'pieces' becomes 'men' (man can mean a chess piece).
'amaze'+'men'='amazemen'
't' after 'amazemen' is 'AMAZEMENT'.
Can you help me to learn more?
(Other definitions for amazement that I've seen before include "Shock" , "Astonished reaction" , "Stupefaction" , "Incredulity" , "Great astonishment" .)