Introduction to Bach's Requiem is orchestrated with enthusiasm (4)
I believe the answer is:
brio
'enthusiasm' is the definition.
(I've seen this before)
'introduction to bach's requiem is orchestrated' is the wordplay.
'introduction to' says to take the initial letters.
The initial letters of 'bachs requiem is orchestrated' is 'BRIO'.
'with' is the link.
(Other definitions for brio that I've seen before include "'Con ..., musically with vigour (4)'" , "Liveliness, maybe in music" , "In music, con ..... means with dash and verve" , "'Liveliness, vivacity (4)'" , "(Of music) spirit" .)