Force, down a shade (4,4)
I believe the answer is:
navy blue
'a shade' is the definition.
(shade of blue)
'force down' is the wordplay.
'force' becomes 'navy' (navy is a kind of force).
'down' becomes 'blue' (both can mean sad).
'navy'+'blue'='NAVY-BLUE'
(Other definitions for navy blue that I've seen before include "Dark colour as worn by officers at sea" , "Colour of British seagoing uniforms" , "Dark shade, worn by mariners?" , "Shade - venal buy (anag)" , "Sailor's uniform colour" .)