Be exhaustive about rear end of old English dog (6)
I believe the answer is:
detail
'be exhaustive about rear' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'end of old english dog' is the wordplay.
'end of' says to take the final letters.
'english' becomes 'E' (abbreviation as in OED).
'dog' becomes 'tail' (synonyms).
The final letter of 'old' is 'd'.
'd'+'e'+'tail'='DETAIL'
Can you help me to learn more?
(Other definitions for detail that I've seen before include "A point or aspect of" , "Small or important part" , "'Small part, particular (6)'" , "Relate minutely" , "soldiers given a special task" .)