Rash son holds former pupil up, and senior ones (4,4)
I believe the answer is:
head boys
'senior ones' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'rash son holds former pupil up' is the wordplay.
'rash' becomes 'heady' (I've seen this in another clue).
'son' becomes 's' (genealogical abbreviation for son).
'holds' is an insertion indicator.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy' - private school pupil).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'ob' back-to-front is 'bo'.
'heady'+'s'='headys'
'headys' placed around 'bo' is 'HEAD BOYS'.
'and' acts as a link.
Can you help me to learn more?