Don't speak — otherwise do speech alone (4,4,5)
I believe the answer is:
hold ones peace
'don't speak' is the definition.
I know nothing about this answer so I can't tell whether this works.
'otherwise do speech alone' is the wordplay.
'otherwise' indicates anagramming the letters.
'do'+'speech'+'alone'='dospeechalone'
'dospeechalone' is an anagram of 'HOLD ONES PEACE'.
Can you help me to learn more?
(Other definitions for hold ones peace that I've seen before include "Keep silent" , "Keep one's thoughts to oneself" .)