Elk has right to be gloomy (6)
I believe the answer is:
morose
'gloomy' is the definition.
(thesaurus)
'elk has right' is the wordplay.
'elk' becomes 'moose' ('moose' can be a synonym of 'elk').
'has' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'moose' enclosing 'r' is 'MOROSE'.
'to be' is the link.
(Other definitions for morose that I've seen before include "Sullen, crabby" , "Miserable" , "Sullen and moody" , "blue" , "Silent, gloomy and ill-tempered" .)