Gosh! US soldier is royal companion? (5)
I believe the answer is:
corgi
'royal companion?' is the definition.
(breed of dog associated with the Queen)
'gosh us soldier' is the wordplay.
'gosh' becomes 'cor' (I've seen this before).
'us soldier' becomes 'gi' (I've seen this in other clues).
'cor'+'gi'='CORGI'
'is' acts as a link.
(Other definitions for corgi that I've seen before include "man's best friend" , "Royal pet" , "Royal canine" , "Pre-2018 royal dog" , "found in the kennels" .)