Not all keep sombre in course (5)
I believe the answer is:
epsom
'course' is the definition.
(I've seen this before)
'not all keep sombre in' is the wordplay.
'not all' indicates a hidden word.
'EPSOM' can be found hidden inside 'keep sombre in'.
(Other definitions for epsom that I've seen before include "Derby and Oaks racecourse" , "Derby course" , "Mares would triumph here" , "Surrey racing town" , "Location of English Derby" .)