Melissa gets bid in though, sadly, it's not allowed (12)
I believe the answer is:
inadmissible
'not allowed' is the definition.
The answer and definition are not the same part of speech. However, past participle verbs and adjectives occasionally define each other.
'melissa gets bid in though sadly it's' is the wordplay.
I cannot really see how this works, but
'gets' could be 'is' (I've seen this in other clues) and 'is' is present in the answer.
'in' is within the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for inadmissible that I've seen is " Not to be tolerated".)