Sound as a bell, old dog (5)
I believe the answer is:
dingo
'dog' is the definition.
(I've seen this before)
'sound as a bell old' is the wordplay.
'sound as a bell' becomes 'ding' (make a ding sound).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ding'+'o'='DINGO'
(Other definitions for dingo that I've seen before include "Australian native" , "Aussie wild dog" , "Want a dog" , "Wild thing" , "Aus. wild dog" .)