Fellow soldier fencing run for dog (5)
I believe the answer is:
corgi
'dog' is the definition.
(corgi is a kind of dog)
'fellow soldier fencing run' is the wordplay.
'fellow' becomes 'co' (eg a co-worker is a fellow worker).
'soldier' becomes 'GI' (GI can mean a US soldier).
'fencing' is an insertion indicator (to fence can mean to contain or put a limit round).
'run' becomes 'r' (cricket abbreviation).
'co'+'gi'='cogi'
'cogi' going around 'r' is 'CORGI'.
'for' is the link.
(Other definitions for corgi that I've seen before include "The Queen's favourite?" , "Welsh breen of dog" , "Royal canine" , "Royal pet" , "Pre-2018 royal dog" .)