A compelling figure in orchestration, Gil Evans (8)
I believe the answer is:
svengali
'a compelling figure' is the definition.
(controlling or mesmerising person)
'in orchestration gil evans' is the wordplay.
'in orchestration' is an anagram indicator (letters orchestrated or arranged a new way).
'gil'+'evans'='gilevans'
'gilevans' is an anagram of 'SVENGALI'.
(Other definitions for svengali that I've seen before include "Evil person - leavings (anag)" , "someone having 5 [INFLUENCED] victims?" , "man of influence" , "George du Maurier's controlling character" , "(controlling person)" .)