Orchestra leader to look over popular overture from Ives (9)
I believe the answer is:
toscanini
'orchestra leader' is the definition.
(Italian conductor Arturo Toscanini)
'to look over popular overture from ives' is the wordplay.
'look over' becomes 'scan' (I've seen this in another clue).
'popular' becomes 'in' ('in' can mean trendy).
'overture from' indicates taking the first letters (the introduction or overture to the word).
The first letter of 'ives' is 'i'.
'to'+'scan'+'in'+'i'='TOSCANINI'
(Other definitions for toscanini that I've seen before include "Music director" , "He conducted" , "Virtuoso conductor" , "Bare rock formation" , "Maestro" .)