Rustless metal fencing sword, very thin (7)
I believe the answer is:
tinfoil
'very thin' is the definition.
I can't tell whether this definition defines the answer.
'rustless metal fencing sword' is the wordplay.
'rustless metal' becomes 'tin' (tin is a kind of metal. I am not sure about the 'rustless' bit.).
'fencing sword' becomes 'foil' (foil is a kind of fencing sword).
'tin'+'foil'='TINFOIL'
Can you help me to learn more?
(Other definitions for tinfoil that I've seen before include "Thin metallic sheet" , "Shiny food wrapping" , "Thin metal sheet for wrapping food etc" , "A help in the kitchen" , "Kitchen item" .)