Lengthy message, lacking opening, reached conclusion (8)
I believe the answer is:
extended
'lengthy' is the definition.
(I've seen this before)
'message lacking opening reached conclusion' is the wordplay.
I cannot really see how this works, but
'message' could be 'ded' (DEd is an example) and 'ded' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for extended that I've seen before include "Prolonged, lengthened" , "Stretched out over a distance" , "Stretched or prolonged" , "Elongated, expanded" , "Stretched out over time or space" .)