Sinister fish, ultimately nasty (5)
I believe the answer is:
fishy
'sinister' is the definition.
(thesaurus)
'fish ultimately nasty' is the wordplay.
'ultimately' indicates one should take the final letters.
The last letter of 'nasty' is 'y'.
'fish'+'y'='FISHY'
(Other definitions for fishy that I've seen before include "Suspicious in piscine way" , "Arousing suspicion" , "Suspicious (as trout?)" , "Questionable" , "Ray is so" .)