Unfortunate amnesia about that dressing (10)
I believe the answer is:
mayonnaise
'dressing' is the definition.
(mayonnaise is a kind of dressing)
'unfortunate amnesia about that' is the wordplay.
'unfortunate' is an anagram indicator.
'about' is an insertion indicator.
'that' becomes 'yon' (archaic term meaning 'that').
'amnesia' with letters rearranged gives 'manaise'.
'manaise' enclosing 'yon' is 'MAYONNAISE'.
(Other definitions for mayonnaise that I've seen before include "Creamy dressing" , "7 for salad" , "which is saucy" , "Salad dressing" , "Egg/oil sauce" .)