Antelope's caught by assistant putting down one lamb (5,3)
I believe the answer is:
agnus dei
'lamb' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'antelope's caught by assistant putting down one' is the wordplay.
'antelope's' becomes 'gnu's' (gnu is a kind of antelope).
'caught by' is an insertion indicator.
'assistant putting down' becomes 'ade' (I am not sure about this - if you are sure you should believe this answer much more).
'one' becomes 'i' (Roman numeral).
'gnus' inserted within 'ade' is 'agnusde'.
'agnusde'+'i'='AGNUS DEI'
Can you help me to learn more?
(Other definitions for agnus dei that I've seen before include "Part of the ordinary" , "religious piece" , "Christian emblem" , "Part of Latin Mass" , "sacrificial offering" .)