COMMON-LISP-USER> (defstruct foo a b c)
FOO
COMMON-LISP-USER> (defstruct (kid (:include foo (b 5) (gaxr 'default))) a-new-slot)
KID
COMMON-LISP-USER> (make-kid)
#S(KID :A NIL :B 5 :C NIL :A-NEW-SLOT NIL)

I got bitten by thinking that a derived structure type was properly changing the defaulting expression for a slot in the ancestor.
CLHS says of:
 (:include included-structure-name slot-description*)
Each slot-description must have a slot-name that is the same as that of some slot in the included structure.