Gee, an American soldier's dog (5)
I believe the answer is:
corgi
'dog' is the definition.
(corgi is a kind of dog)
'gee an american soldier's' is the wordplay.
'gee an' becomes 'cor' (I can't justify this - if you can you should believe this answer much more).
'american soldier' becomes 'GI' (informal term for a US soldier).
'cor'+'gi'='CORGI'
Can you help me to learn more?
(Other definitions for corgi that I've seen before include "Small Welsh dog with fox-like head" , "Welsh breed of short-legged dog" , "Queen likes this" , "Little dog" , "Welsh 11" .)