Expression of surprise over US soldier’s dog (5)
I believe the answer is:
corgi
'dog' is the definition.
(I know that corgi is a type of dog)
'expression of surprise over us soldier's' is the wordplay.
'expression of surprise' becomes 'cor' ('cor!' is an expression of surprise).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'us soldier' becomes 'GI' (GI can mean an American soldier).
'cor'+'gi'='CORGI'
(Other definitions for corgi that I've seen before include "Welsh 11" , "Royal canine" , "Pre-2018 royal dog" , "Cardigan, possibly" , "found in the kennels" .)