Say — say indistinctly, hiding head (5)
I believe the answer is:
utter
'say say' is the definition.
(saying is a kind of uttering)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for utter that I've seen before include "Make (a sound); complete" , "Speak - it's extreme" , "Speak, articulate" , "Perfect" , "Total - speak" .)