Old canine barking threateningly at first (7)
I believe the answer is:
ancient
'old' is the definition.
(similar in meaning)
'canine barking threateningly at first' is the wordplay.
'barking' is an anagram indicator (UK slang for mad).
'at first' says to take the initial letters.
The first letter of 'threateningly' is 't'.
'canine' is an anagram of 'ancien'.
'ancien'+'t'='ANCIENT'
(Other definitions for ancient that I've seen before include "of long standing" , "Very old or belonging to times past" , "Belonging to a former age" , "no longer current" , "Shakespearean standard-bearer" .)