Yet all thought must be curtailed (8)
I believe the answer is:
although
'yet' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for although that I've seen before include "Yet" , "Notwithstanding" , "Despite the fact that (8)" , "However, but" , "Apart from the fact that" .)