Get up about noon and use mouthwash? (5)
I believe the answer is:
rinse
'use mouthwash?' is the definition.
('rinse' can be a synonym of 'use mouthwash')
'get up about noon' is the wordplay.
'get up' becomes 'rise' (synonyms).
'about' is an insertion indicator.
'noon' becomes 'n' (abbreviation).
'rise' placed around 'n' is 'RINSE'.
'and' acts as a link.
(Other definitions for rinse that I've seen before include "Wash in clean water" , "Wash off surface dirt" , "Wash lightly without soap" , "Use water to remove soap" , "Colour" .)