Gloomy American elk? That's about right (6)
I believe the answer is:
morose
'gloomy' is the definition.
(I've seen this before)
'american elk? that's about right' is the wordplay.
'american elk?' becomes 'moose' (I've seen this before).
'that's about' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'moose' enclosing 'r' is 'MOROSE'.
(Other definitions for morose that I've seen before include "Sullen and moody" , "Sullen and gloomy" , "dark" , "in gloomy mood" , "Peevish and gloomy" .)