Stout knocked back with cheese? That's the stuff! (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')
'stout knocked back with cheese?' is the wordplay.
'stout' becomes 'fat' ('fat' can be similar in meaning to 'stout').
'knocked back' is a reversal indicator.
'with' is a charade indicator (letters next to each other).
'cheese?' becomes 'feta' (I've seen this before).
'fat' in reverse letter order is 'taf'.
'taf'+'feta'='TAFFETA'
'that's' acts as a link.
(Other definitions for taffeta that I've seen before include "Sheeny fabric" , "fine stuff indeed!" , "Lustrous silky fabric" , "A shiny silklike fabric" , "stuff made for dressing" .)