Endless support in part that is gloomy (6)
I believe the answer is:
sombre
'gloomy' is the definition.
(I've seen this before)
'endless support in part' is the wordplay.
'endless' means to remove the last letter.
'support' becomes 'bra' (a support for the breasts).
'in' indicates putting letters inside.
'part' becomes 'some' (I've seen this before**).
'bra' with its final letter removed is 'br'.
'br' going into 'some' is 'SOMBRE'.
'that is' acts as a link.
(Other definitions for sombre that I've seen before include "Dull, gloomy" , "Dark, dull" , "Dark, serious, gloomy" , "Lugubrious" , "Solemn and grave" .)