Got alibi? Because at first they can't be omitted (9)
I believe the answer is:
obbligati
'they can't be omitted' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'got alibi? because at first' is the wordplay.
I cannot quite understand how this works, but
'at' is present in the answer.
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for obbligati that I've seen before include "Musical parts not to be omitted" , "being essential to score" , "Instrumental parts not to be omitted" .)