Flat plane piece used in surfing (3)
I believe the answer is:
fin
This clue might be a double definition.
'flat plane piece' is the first definition.
'fin' can be an answer for 'piece' (fin is a kind of piece). I am not sure about the 'flat plane' bit.
'surfing' is the second definition.
(thesaurus)
'used in' is the link.
Can you help me to learn more?
(Other definitions for fin that I've seen before include "Swimmer" , "For movement and balance in fish" , "Fishy appendage" , "Part of an aircraft's tail" , "Paddle-like organ" .)