Son leaving arrangement under his sinister influence (8)
I believe the answer is:
svengali
'sinister influence' is the definition.
(thesaurus)
'son leaving arrangement under' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'arrangement' indicates an anagram.
'under' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'leaving' is an anagram of 'vengali'.
's'+'vengali'='SVENGALI'
'his' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for svengali that I've seen before include "Leavings of a fictional evil hypnotist" , "man of influence" , "Sinister controller (of another person)" , "(controlling person)" , "Evil person - leavings (anag)" .)