Tavern with relative amount of beer (6)
I believe the answer is:
barrel
'amount of beer' is the definition.
'barrel' can be an answer for 'amount' (barrel is a kind of amount). I am unsure of the 'of beer' bit.
'tavern with relative' is the wordplay.
'tavern' becomes 'bar' (I've seen this in another clue).
'with' says to put letters next to each other.
'relative' becomes 'rel' (mathematical abbreviation).
'bar'+'rel'='BARREL'
Can you help me to learn more?
(Other definitions for barrel that I've seen before include "''Roll out the ..., let's have a ... of fun'' (WWII song)" , "Wood vessel; part of horse" , "Round wooden vessel." , "Cylindrical crate" , "needs some oil" .)