Fabric in Welsh river with letter from Greece pinned to it (7)
I believe the answer is:
taffeta
'fabric' is the definition.
(I know that taffeta is a type of fabric)
'welsh river with letter from greece pinned to it' is the wordplay.
I cannot really see how this works, but
'river' could be 'taff' (river in Wales) and 'taff' is located in the answer.
'letter' could be 'eta' (eta is a kind of letter) and 'eta' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for taffeta that I've seen before include "fine material for dressing" , "Shiny silk fabric" , "Stiff shiny material" , "Thin glossy silk fabric" , "Shiny silk with a crisp texture" .)