Old canine astonishingly beginning to talk (7)
I believe the answer is:
ancient
'old' is the definition.
(similar in meaning)
'canine astonishingly beginning to talk' is the wordplay.
'astonishingly' indicates an anagram.
'beginning to' indicates taking the first letters.
The initial letter of 'talk' is 't'.
'canine' is an anagram of 'ancien'.
'ancien'+'t'='ANCIENT'
(Other definitions for ancient that I've seen before include "of long standing" , "Shakespearean standard-bearer" , "from Aristophanes' time" , "showing signs of wear?" , "Old salt in Coleridge's poem" .)