Old canine out beginning to tire (7)
I believe the answer is:
ancient
'old' is the definition.
(similar in meaning)
'canine out beginning to tire' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'beginning to' suggests taking the first letters.
The initial letter of 'tire' is 't'.
'canine' anagrammed gives 'ancien'.
'ancien'+'t'='ANCIENT'
(Other definitions for ancient that I've seen before include "Archaic" , "so old-fashioned" , "Antediluvian" , "From the distant past" , "Old man" .)