My grunt for dog? (5)
I believe the answer is:
corgi
'for dog?' is the definition.
(corgi is a kind of dog)
'my grunt' is the wordplay.
'my' becomes 'cor' (both are expressions of surprise).
'grunt' becomes 'gi' (both can mean a US soldier).
'cor'+'gi'='CORGI'
(Other definitions for corgi that I've seen before include "Cardigan, possibly" , "The Queen's favourite?" , "Pre-2018 royal dog" , "Welsh breen of dog" , "Barker?" .)