Mostly gag most about loud emitter of much gas? (7)

'emitter of much gas?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'mostly gag most about loud' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'most' becomes 'best' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'about' indicates putting letters inside.
'loud' becomes 'f' (short for forte - 'loud' in music).
'gag' with its final letter taken away is 'ga'.
'best' placed around 'f' is 'bfest'.
'ga'+'bfest'='GABFEST'

Can you help me to learn more?