Third child in eighth group (4)
I believe the answer is:
seth
'third child' is the definition.
(I've seen this before)
'eighth group' is the wordplay.
I cannot really see how this works, but
'group' could be 'set' (both can mean a collection) and 'set' is found within the answer.
A single letter 'h' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for seth that I've seen before include "Indian novelist, Vikram" , "Adam and Eve's third child" , "Biblical character" , "Indian writer" , "One of the first men" .)