Clue: Swimmer shockingly flashing around pier – not very good (10)
Answer: ANGLERFISH
Explanation: Definition: Swimmer
 **Anagram of**(shockingly) FLASHING **containing**(around) "pier" **minus**(not) "pi"(short for "pious"/very good).
That "light bulb" at the top of its head is its bait used to fish for other fish: