Marc,

  > I need to define an inheritance behavior that concatenates string-values
  > of superclass-initforms with local slot initforms. E.g.
  > 
  > (defclass super()     ((f :accessor f :initform "head")) (:metaclass
  > user-class))
  > (defclass sub(super)  ((f :accessor f :initform "tail")) (:metaclass
  > user-class))
  > 
  > I'd like to get the following:
  > 
  > (f(make-instance'sub)) -> "head tail"
  > 
  > 
  > I didn't find a standard option in defclass slot-descriptions for this.
  > I'd like to define the concatenate combination for each meta-class
  > 'user-class'.
  > 
  > Any idea? Thanks in advance!

try this:

(defclass user-class (standard-class) ())

(defmethod clos:compute-effective-slot-definition
    ((the-class user-class) slot-name
     ;; The order of the direct slots in direct-slot-definitions may
     ;; be reversed in other LISPs (this is code written & tested with
     ;; ACL 4.3):
     direct-slot-definitions)
  (let ((slot-definition (call-next-method))
	(new-initform nil))
    (loop for slot in direct-slot-definitions
	as initform = (clos:slot-definition-initform slot)
	when (stringp initform)
	do
	  ;; Collecting the result string could be done perhaps more
	  ;; elegant:
	  (setf new-initform (if new-initform
				 (concatenate 'string initform " "
					      new-initform)
			       initform)))
    (when new-initform
      ;; Since at (call-next-method) both the initform and
      ;; initfunction of the effective-slot had been set, both must be
      ;; changed here, too:
      (setf (slot-value slot-definition 'clos::initform) new-initform)
      (setf (slot-value slot-definition 'clos::initfunction)
	(constantly new-initform)))
    slot-definition))

(defclass super ()
  ((f :accessor f :initform "head"))
  (:metaclass user-class))

(defclass sub(super)
  ((f :accessor f :initform "tail"))
  (:metaclass user-class))

(f (make-instance 'sub))
==> "head tail"

-- 
Viele Gruesse, Heiko