Goodness me! Soldier becomes royal pet! (5)
I believe the answer is:
corgi
'royal pet' is the definition.
(I've seen this before)
'goodness me soldier' is the wordplay.
'goodness me' becomes 'cor' (both are expressions of surprise).
'soldier' becomes 'GI' (GI can mean a US soldier).
'cor'+'gi'='CORGI'
'becomes' is the link.
(Other definitions for corgi that I've seen before include "Small Welsh dog with fox-like head" , "Cardigan, possibly" , "Toy car manufacturer" , "Barker?" , "Queen's dog?" .)