Somewhat polite, mannerly, couple (4)
I believe the answer is:
item
'couple' is the definition.
(I've seen this before)
'somewhat polite mannerly' is the wordplay.
'somewhat' indicates a hidden word.
'ITEM' is hidden in the letters of 'polite mannerly'.
(Other definitions for item that I've seen before include "Romantic couple" , "One unit on a list" , "One of a series (or a couple!)" , "Loving couple" , "Separate object or unit" .)