They worried about involving head of music with operetta (3,5,5)
I believe the answer is:
the merry widow
'operetta' is the definition.
(I've seen this before)
'they worried about involving head of music with' is the wordplay.
I cannot really understand how this works, but
'of' could be 'o' and 'o' is present in the answer.
'music' could be 'theme' (theme is a kind of music) and 'theme' is located in the answer.
'with' could be 'w' (abbreviation) and 'w' is found within the answer.
This explanation may well be incorrect...
(Other definitions for the merry widow that I've seen before include "show" , "Franz Lehar operetta" .)