Soup book is rather lacking in sex appeal (6)
I believe the answer is:
bisque
'soup' is the definition.
(bisque is a kind of soup)
'book is rather lacking in sex appeal' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'rather' becomes 'quite' (synonyms).
'lacking in' indicates named letters should be taken away.
'sex appeal' becomes 'it'.
'quite' with 'it' taken away is 'que'.
'b'+'is'+'que'='BISQUE'
(Other definitions for bisque that I've seen before include "Rich shellfish soup - lobster is nice" , "Thick shellfish soup" , "pot?" , "Rich soup as made from lobster" , "Shellfish soup - kind of pottery" .)