Capital but short name for dog (5)
I believe the answer is:
cairn
'for dog' is the definition.
(cairn is a kind of dog)
'capital but short name' is the wordplay.
'capital' becomes 'cairo' (capital of Egypt).
'but short' means to remove the last letter (the word is short of its final letter).
'name' becomes 'n' (common abbreviation e.g. on forms).
'cairo' with its last letter taken away is 'cair'.
'cair'+'n'='CAIRN'
(Other definitions for cairn that I've seen before include "Man" , "Mound of stones erected as marker" , "Rocky pile" , "Memorial mound of stones" , "dog" .)