Welsh runner on return put away stuff (7)
I believe the answer is:
taffeta
'stuff' is the definition.
(I have seen 'silky stuff' mean 'taffeta' so perhaps 'stuff' could also mean 'taffeta')
'welsh runner on return put away' is the wordplay.
'welsh runner' becomes 'Taff' (Welsh river - something that 'runs').
'on return' says the letters should be written backwards.
'put away' becomes 'ate' (I've seen this in another clue).
'ate' reversed gives 'eta'.
'taff'+'eta'='TAFFETA'
(Other definitions for taffeta that I've seen before include "cloth" , "Crisp, smooth, lustrous fabric" , "Shiny silk with a crisp texture" , "Lustrous silky fabric" , "silky stuff" .)