They can see the Speaker's support for motion (4)
I believe the answer is:
eyes
'they can see' is the definition.
'the speaker's support for motion' is the wordplay.
'the speaker's' indicates a 'sounds like' (homophone) clue.
'support for motion' becomes 'ayes' (votes in favour in eg UK parliament).
'ayes' is a homophone of 'EYES'.
(Other definitions for eyes that I've seen before include "Peacock's tail markings" , "''Drink to me only with thine ...'' (Johnson)" , "Are they made to suggest possible date" , "Visual organs" , "They look" .)