In Scotland, hit old wild dog (5)
I believe the answer is:
dingo
'wild dog' is the definition.
(dingo is a kind of wild dog)
'in scotland hit old' is the wordplay.
'in scotland hit' becomes 'ding' (Scottish term for a hit on the head).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ding'+'o'='DINGO'
(Other definitions for dingo that I've seen before include "Wild thing" , "One running wild in Australia" , "Kipling's "Yellow-Dog"" , "dog running wild" , "Wild creature" .)