Adam's son, determined to get a goal on the rugby field (4)
I believe the answer is:
seth
'adam's son' is the definition.
(I've seen this in another clue)
'determined to get a goal on the rugby field' is the wordplay.
'determined' becomes 'set' (synonyms).
'to get' is a charade indicator (letters next to each other).
'a goal on the rugby field' becomes 'h' (I can't explain this - if you can you should give a lot more credence to this answer).
'set'+'h'='SETH'
Can you help me to learn more?
(Other definitions for seth that I've seen before include "Boy's name" , "god of Egypt" , "Biblical character" , "Indian writer" , "In disguise, the brother of Cain and Abel" .)