He's determined to get a goal at Twickenham (4)
I believe the answer is:
'he's' is the definition.
'determined to get a goal at twickenham' is the wordplay.
I cannot really understand how this works, but
'determined' could be 'set' (synonyms) and 'set' is found in the answer.
A single letter 'h' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for seth that I've seen before include "One of the first names recorded" , "Adam's third son" , "Boy" , "Boy's name" , "Adam and Eve's third child" .)