Bar puts something unknown in beer (4)
I believe the answer is:
axle
'bar' is the definition.
(I've seen this before)
'something unknown in beer' is the wordplay.
'something unknown' becomes 'x' (an unknown value in algebra).
'in' indicates putting letters inside.
'beer' becomes 'ale' (type of beer).
'x' going into 'ale' is 'AXLE'.
'puts' is the link.
(Other definitions for axle that I've seen before include "Mandrel" , "Shaft on which a wheel rotates" , "Wheel pivot" , "Rotating rod carrying wheels" , "Rotating shaft" .)