Suspect, unlikely (5)
I believe the answer is:
fishy
This clue is a double definition.
'suspect' is the first definition.
(I know that suspect can be written as fishy)
'unlikely' is the second definition.
(thesaurus)
(Other definitions for fishy that I've seen before include "Suspect" , "As smelt, perhaps" , "Suspicious (as trout?)" , "Suspicious in piscine way" , "Arousing suspicion" .)