If upset, nervous, suspicious (5)
I believe the answer is:
fishy
'suspicious' is the definition.
(I know that suspicious can be written as fishy)
'if upset nervous' is the wordplay.
'upset' says the letters should be written backwards.
'nervous' becomes 'shy' (I've seen this in other clues).
'if' in reverse letter order is 'fi'.
'fi'+'shy'='FISHY'
(Other definitions for fishy that I've seen before include "Arousing suspicion" , "Suspect" , "Questionable" , "Ray is so" , "Suspicious (as trout?)" .)