Frothy stuff provided by helpless pub server (4)

'server' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'frothy stuff provided by helpless pub' is the wordplay.
'frothy' becomes 'foaming' (I've seen this before).
'stuff' means to look at the middle letters (I've seen 'stuffing' mean this).
'provided by' says to put letters next to each other.
'helpless pub' becomes 'bar' (both are drinking establishments. I am not sure about the 'helpless' bit.).
The centre of 'foaming' is 'm'.
'm' put after 'bar' is 'BARM'.

Can you help me to learn more?