One laboriously sought at the centre? (3)
I believe the answer is:
job
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'laboriously sought at the centre?' is the wordplay.
'laboriously' becomes 'jo' (I can't explain this - if you can you should believe this answer much more).
'sought' becomes 'grubbed' (grubbing is a kind of seeking).
'at the centre?' means to look at the middle letters.
The centre of 'grubbed' is 'b'.
'jo'+'b'='JOB'
Can you help me to learn more?
(Other definitions for job that I've seen before include "Assignment" , "Employment" , "task for Hebrew leader noted for misfortunes" , "Book; task" , "Personification of patience" .)