Communication causes a mix-up a long time after (7)
I believe the answer is:
message
'communication' is the definition.
(I know that communication can be written as message)
'mix-up a long time after' is the wordplay.
'mix-up' becomes 'mess' (I've seen this before).
'a long time' becomes 'age' (an age is a period of time).
'after' is a charade indicator (letters next to each other).
'mess'+'age'='MESSAGE'
'causes a' acts as a link.
(Other definitions for message that I've seen before include "meaning" , "Brief communication" , "Missive" , "Signal" , "Spoken or written communication" .)