Extremely concerned about upper-class celebrity’s sauce (7)
I believe the answer is:
custard
'sauce' is the definition.
(I've seen this before)
'extremely concerned about upper-class celebrity's' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'about' indicates putting letters inside.
'upper class' becomes 'u' (as in U- and non-U speech).
'celebrity's' becomes 'star' (I've seen this before).
'concerned' with its centre removed is 'cd'.
'u'+'star'='ustar'
'cd' going around 'ustar' is 'CUSTARD'.
(Other definitions for custard that I've seen before include "Yellow mixture" , "Type of apple" , "Pudding sauce" , "Dessert sauce" , "Pudding accompaniment" .)