The conjectural conclusion is about 1500 miles long! (4)
I believe the answer is:
ural
'1500 miles long' is the definition.
I can't tell whether this definition defines the answer.
'the conjectural conclusion is about' is the wordplay.
'is about' indicates a hidden word (the clue goes around or about the answer).
'URAL' is hidden in the letters of 'conjectural conclusion'.
Can you help me to learn more?
(Other definitions for ural that I've seen before include "Russian river flowing into the Caspian Sea" , "Asian flower" , "2,400 km river flowing through Russia and Kazakhstan into the Caspian Sea" , "River feeding Caspian Sea" , "Mountain range in Russia" .)