The stuff the terribly fat fellow ate! Weird! (7)
I believe the answer is:
taffeta
'the stuff' is the definition.
(I have seen 'silky stuff' mean 'taffeta' so perhaps 'stuff' could also mean 'taffeta')
'the terribly fat fellow ate weird' is the wordplay.
'the terribly' indicates anagramming the letters.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'weird' is an anagram indicator.
'fat' anagrammed gives 'taf'.
'ate' is an anagram of 'eta'.
'taf'+'f'+'eta'='TAFFETA'
(Other definitions for taffeta that I've seen before include "it has a fine texture" , "Shiny silk with a crisp texture" , "Sheeny fabric" , "Thin glossy silk fabric" , "Lustrous silky fabric" .)