Audible cargo said "moo" (5)
I believe the answer is:
lowed
'said moo' is the definition.
The definition and answer can be both to do with communicating as well as being past participle verbs.
Maybe you can see a link between them that I can't see?
'audible cargo' is the wordplay.
'audible' shows a homophone (sound like).
'cargo' becomes 'load' (synonyms).
'load' sounds like 'LOWED'.
Can you help me to learn more?
(Other definitions for lowed that I've seen before include "Mooed" , "made noises like farm animals" , "How Jersey communicated" .)