Flat object used in surfing (3)
I believe the answer is:
fin
'surfing' is the definition.
(thesaurus)
'flat object' is the wordplay.
'flat' becomes 'fi' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'object' becomes 'n' (N is an example).
'fi'+'n'='FIN'
'used in' is the link.
Can you help me to learn more?
(Other definitions for fin that I've seen before include "Part of an aircraft's tail" , "Fish propeller; end (Fr.)" , "Piscine appendage" , "For movement and balance in fish" , "Fishy appendage" .)