Concern expressed with a later surprised comment? That’s rich (6)
I believe the answer is:
creamy
'rich' is the definition.
(as in rich or creamy foods)
'concern expressed with a later surprised comment?' is the wordplay.
I cannot really understand how this works, but
'a' is found in the answer.
'surprised comment?' could be 'my' (expression of surprise) and 'my' is present in the answer.
This explanation may well be incorrect...
'that's' acts as a link.
(Other definitions for creamy that I've seen before include "rich and white" , "Thickly rich in texture" , "ivory" , "Buttery" , "Rich like fatty part of milk" .)