One in Temple Bar knocked back first couple of bitters (5)
I believe the answer is:
rabbi
'couple of bitters' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'one in temple bar knocked back first' is the wordplay.
I cannot really see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
an anagram of 'bar' is 'rab' which is located in the answer.
'back first' could be 'b' (1st letter of 'back') and 'b' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rabbi that I've seen before include "A Jewish clergyman" , "Jewish law expounder" , "legal scholar" , "Teacher of Jewish law" , "Blue" .)