Where balls are to cast up in rugby formation (7)
I believe the answer is:
scrotum
'where balls' is the definition.
'scrotum' can be an answer for 'balls' (thesaurus). I'm not sure about the 'where' bit.
'to cast up in rugby formation' is the wordplay.
'to cast up' becomes 'ot' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'rugby formation' becomes 'scrum' (I've seen this in another clue).
'ot' put within 'scrum' is 'SCROTUM'.
'are' is the link.
Can you help me to learn more?
(Other definitions for scrotum that I've seen before include "Wrinkled retainer" , "manbag?" , "man's bag" .)