Material gains for Grand National jockey (6)
I believe the answer is:
velvet
'material' is the definition.
(I know that velvet is a type of material)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for velvet that I've seen before include "Dense silky fabric" , "A glove made of this might cover an iron hand" , "Fabric with thick soft pile" , "Silky fabric with thick pile" , "Silky, densely-piled fabric" .)