Arrest former pupil, a rich and powerful man (5)
I believe the answer is:
nabob
'powerful man' is the definition.
(I've seen this before)
'arrest former pupil' is the wordplay.
'arrest' becomes 'nab' (synonyms).
'former pupil' becomes 'OB' (abbreviation for 'Old Boy' - private school pupil).
'nab'+'ob'='NABOB'
'a rich and' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for nabob that I've seen before include "very wealthy man" , "Mogul governor; rich man" , "Powerful man" , "Devotional painting" , "Fat cat: governor in India during the Mogul empire" .)