Book is rather lacking that certain something - that’s a handicap (6)
I believe the answer is:
bisque
'a handicap' is the definition.
(extra turn taken by a weaker player in croquet)
'book is rather lacking that certain something' is the wordplay.
'book' becomes 'bis' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'is rather' becomes 'quite' (I've seen this before).
'lacking' is a deletion indicator.
'that certain something' becomes 'it' (eg a pop star may be said to have 'it').
'quite' with 'it' taken away is 'que'.
'bis'+'que'='BISQUE'
'that's' is the link.
(Other definitions for bisque that I've seen before include "Rich shellfish soup - lobster is nice" , "pot?" , "Rich soup made from shellfish" , "Rich soup as made from lobster" , "Creamy soup" .)