Goodness! Soldier becomes royal pet! (5)
I believe the answer is:
corgi
'royal pet' is the definition.
(I've seen this before)
'goodness soldier' is the wordplay.
'goodness' becomes 'cor' (both are exclamations of surprise).
'soldier' becomes 'GI' (term for US soldier).
'cor'+'gi'='CORGI'
'becomes' is the link.
(Other definitions for corgi that I've seen before include "Welsh 11" , "Royal canine" , "Little dog" , "Queen's dog?" , "Dog of Welsh breed, royal favourite" .)