Upset: tartlet endlessly curdles (6)
I believe the answer is:
rattle
'upset tartlet endlessly curdles' is the definition.
I can't judge whether this definition defines the answer.
'upset tartlet endlessly' is the wordplay.
'upset' indicates an anagram.
'endlessly' means to remove the last letter.
'tartlet' with its last letter taken off is 'tartle'.
'tartle' with letters rearranged gives 'RATTLE'.
Can you help me to learn more?
(Other definitions for rattle that I've seen before include "Noisy type of snake" , "making one upset" , "Baby's toy; sound of loose part" , "Baby's toy; sound of something loose" , "Child's noisy toy" .)