Interpret mouth movements (3-4)
I believe the answer is:
lip-read
'interpret mouth movements' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lip-read that I've seen before include "Interpret someone''s words when deaf" , "Understand by watching (a person) speak" , "Dire pal can interpret mouth movements as words" , "Understand without hearing" , "How to "hear" when deaf" .)