Drum along with mildly vacuous rock opera (5)
I believe the answer is:
tommy
'rock opera' is the definition.
(I know that Tommy is a rock opera)
'drum along with mildly vacuous' is the wordplay.
'drum' becomes 'tom' (short for tom-tom).
'along with' says to put letters next to each other.
'vacuous' says to hollow out the word (remove centre letters).
'mildly' with its middle removed is 'my'.
'tom'+'my'='TOMMY'
(Other definitions for tommy that I've seen before include "Doubtful apostle could be so familiar" , "British private soldier (colloq.)" , "Private in the Army - kind of gun" , "Bread" , "British soldier (informal)" .)