Gloomy elk crossing river (6)
I believe the answer is:
morose
'gloomy' is the definition.
(I've seen this in another clue)
'elk crossing river' is the wordplay.
'elk' becomes 'moose' (synonyms).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'river' becomes 'r' (abbreviation for river).
'moose' going around 'r' is 'MOROSE'.
(Other definitions for morose that I've seen before include "in a grumpy mood" , "Sullen and brooding" , "Miserable" , "Peevish and gloomy" , "Sullen and moody" .)