Actor Matthau (6)
I believe the answer is:
'actor matthau' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for walter that I've seen before include "Elizabethan navigator" , "Sir . . . . . . Scott, famous Scottish poet and novelist of 'Ivanhoe' etc" , "Sir --- 23, novelist" , "Sir - Scott; - de la Mare" , "- de la Mare, English poet" .)