Long message, missing opening, came to a conclusion (8)
I believe the answer is:
extended
'long' is the definition.
('extended' can be similar in meaning to 'long')
'message missing opening came to a conclusion' is the wordplay.
'message' becomes 'text' (I've seen this before).
'missing opening' means to remove the first letter.
'came to a conclusion' becomes 'ended'.
'text' with its initial letter taken away is 'ext'.
'ext'+'ended'='EXTENDED'
(Other definitions for extended that I've seen before include "get out" , "Stretched, drew out" , "Lengthy" , "Made longer" , "Stretched out over time or space" .)