Highland beer (about 426ml) (6)
I believe the answer is:
alpine
'highland' is the definition.
(similar in meaning)
'beer about 426ml' is the wordplay.
'beer' becomes 'ale' (type of beer).
'about' is an insertion indicator.
'4i6ml' becomes 'pin' (I can't justify this - if you can you should give a lot more credence to this answer).
'ale' going around 'pin' is 'ALPINE'.
Can you help me to learn more?
(Other definitions for alpine that I've seen before include "Relating to a European mountain range" , "Nepali (anag.)" , "Relating to high mountains" , "from Germany, Austria etc" , "Plant of high mountains" .)