American animal, sullen? Not right (5)
I believe the answer is:
moose
'american animal' is the definition.
(US term for an elk)
'sullen? not right' is the wordplay.
'sullen?' becomes 'morose' (both mean miserable).
'not' is a deletion indicator.
'right' becomes 'r' (common abbreviation).
'morose' with 'r' taken out is 'MOOSE'.
(Other definitions for moose that I've seen before include "beasts" , "Large N American deer like European elk" , "Large northern deer" , "North American elk" , "Ungulate" .)