Legal stipulation can have limits after trial starts (6)
I believe the answer is:
caveat
'legal stipulation can' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'limits after trial starts' is the wordplay.
'limits' becomes 'cave' (I can't explain this - if you can you should give a lot more credence to this answer).
'starts' says to take the initial letters.
The first letters of 'after trial' is 'at'.
'cave'+'at'='CAVEAT'
'have' acts as a link.
Can you help me to learn more?
(Other definitions for caveat that I've seen before include "At cave (anag.)" , "A notice or warning" , "'Caution, warning (6)'" , "'Warning, proviso (6)'" , "A warning against certain acts" .)