There's something fishy about this sign (6)
I believe the answer is:
pisces
'this sign' is the definition.
(Pisces is an example)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for pisces that I've seen before include "The twelfth sign of the zodiac, the fishes" , "Fish star sign" , "'Sign of the zodiac, the fish (6)'" , "Twelfth sign of the zodiac, the Fish" , "Spice for the Fish" .)