Smooth navy blue covers (4)
I believe the answer is:
sand
'smooth' is the definition.
(I know that sand is a more specific form of the action smooth)
'navy blue covers' is the wordplay.
'navy' becomes 'N' (common abbreviation eg in RN Royal Navy).
'blue' becomes 'sad' (similar in meaning).
'covers' means one lot of letters goes inside another.
'n' inserted into 'sad' is 'SAND'.
(Other definitions for sand that I've seen before include "Rock particles; smooth" , "George -, Fr. author" , "Parts of beach" , "Desert grit" , "George -, nineteenth century French novelist" .)