One giving party among toffs' set, so hard up (7)
I believe the answer is:
hostess
'one giving party' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'among toffs' set so hard up' is the wordplay.
'among' indicates the answer is hidden within the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'ssetsoh' is hidden in the letters of 'toffs set so hard'.
'ssetsoh' backwards is 'HOSTESS'.
Can you help me to learn more?
(Other definitions for hostess that I've seen before include "Plane stewardess" , "Woman giving a party" , "Female entertainer" , "Air stewardess" , "Aircraft stewardess" .)