In Herculaneum I'm inside sewer, doomed (4)
I believe the answer is:
mimi
'sewer doomed' is the definition.
(doomed seamstress in opera La boheme)
'in herculaneum i'm inside' is the wordplay.
'in' indicates a hidden word.
'MIMI' is hidden in the letters of 'herculaneum im inside'.
(Other definitions for mimi that I've seen before include "Her tiny hand was frozen (Puccini)" , "Starving character of "La Boheme"" , "I'm upset twice by the operatic girl" , "Puccini heroine" , "Tragic heroine" .)