Ancient worker an experienced one (3,4)
I believe the answer is:
old hand
'an experienced one' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'ancient worker' is the wordplay.
'ancient' becomes 'old' (similar in meaning).
'worker' becomes 'hand' (hand is a kind of worker).
'old'+'hand'='OLD HAND'
Can you help me to learn more?
(Other definitions for old hand that I've seen before include "Experienced or well-practised person" , "Person with a lot of experience" , "expert?" , "Experienced or practised person - manually aged?" , "Gothic script" .)