Sinister influence of seconds, see, on English boxer (8)
I believe the answer is:
svengali
'sinister influence' is the definition.
(thesaurus)
'seconds see on english boxer' is the wordplay.
'seconds' becomes 's'.
'see' becomes 'v' (short for 'vide', Latin meaning 'see elsewhere').
'on' is a charade indicator (letters next to each other).
'english' becomes 'eng' (abbreviation).
'boxer' becomes 'Ali' (Muhammad Ali).
's'+'v'+'eng'+'ali'='SVENGALI'
'of' acts as a link.
(Other definitions for svengali that I've seen before include "Control freak" , "Evil manager" , "Complete controller" , "Person who tries to force another to do his bidding" , "Master manipulator" .)