Warnings that beer only comes by half quarts (6)
I believe the answer is:
alerts
'warnings that beer only' is the definition.
'alerts' can be an answer for 'warnings' (alert is a kind of warning). I'm unsure of the rest of the definition.
'by half quarts' is the wordplay.
'by' becomes 'ale' (I can't justify this - if you can you should believe this answer much more).
'half' means to take half the letters.
'quarts' cut in half is 'rts'.
'ale'+'rts'='ALERTS'
'comes' is the link.
Can you help me to learn more?
(Other definitions for alerts that I've seen before include "danger-signals" , "Alarms, warning signals" , "Warnings" , "Warns, draws attention to" , "Puts on guard, forewarns" .)