Singer nailing musical's opening, quiver (7)
I believe the answer is:
tremble
'quiver' is the definition.
(I've seen this before)
'singer nailing musical's opening' is the wordplay.
'singer' becomes 'treble' (I've seen this in other clues).
'nailing' means one lot of letters goes inside another.
'opening' says to take the initial letters.
The first letter of 'musicals' is 'm'.
'treble' placed around 'm' is 'TREMBLE'.
(Other definitions for tremble that I've seen before include "Show fear" , "Shake with fear or excitement" , "Betray nerves" , "Quakers do this" , "Quake in one's shoes" .)