Expression of surprise over serving American's dogs (6)
I believe the answer is:
corgis
'dogs' is the definition.
(corgi is a kind of dog)
'expression of surprise over serving american's' is the wordplay.
'expression of surprise' becomes 'cor' (I've seen this before).
'over' says to put letters next to each other.
'serving american's' becomes 'gis' (I've seen this in other clues).
'cor'+'gis'='CORGIS'
(Other definitions for corgis that I've seen before include "Palace dogs" , "Welsh breeds of long-bodied short-legged dogs" .)