Attendant about to upset soup (6)
I believe the answer is:
potage
'soup' is the definition.
(potage is a kind of soup)
'attendant about to upset' is the wordplay.
'attendant' becomes 'page' (page is a kind of attendant**).
'about' is an insertion indicator.
'upset' is a reversal indicator.
'to' in reverse letter order is 'ot'.
'page' enclosing 'ot' is 'POTAGE'.
(Other definitions for potage that I've seen before include "that's thick" , "French soup, usually thick" , "Stock food item" , "Thick, often creamy soup" , "which is fairly thick" .)