Speak — do so quietly with face hidden (5)
I believe the answer is:
utter
'speak' is the definition.
(I know that speak can be written as utter)
'do so quietly with face hidden' is the wordplay.
I cannot really understand how this works, but
'do' could be 'ut' (synonyms) and 'ut' is found in the answer.
The remaining letters 'ter' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for utter that I've seen before include "Positive" , "Speak, articulate" , "issue" , "Speak in absolute way" , "Total < state" .)