Surly animal crossing river (6)
I believe the answer is:
morose
'surly' is the definition.
(thesaurus)
'animal crossing river' is the wordplay.
I cannot really see how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is present in the answer.
'animal' could be 'moose' (type of animal) and 'moose' is present in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
(Other definitions for morose that I've seen before include "Sullen and gloomy" , "Miserable" , "Silent, gloomy and ill-tempered" , "in gloomy mood" , "blue" .)