Warns beer froths regularly (6)
I believe the answer is:
alerts
'warns' is the definition.
(alerting is a kind of warning)
'beer froths regularly' is the wordplay.
'beer' becomes 'ale' (type of beer).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'froths' are 'rts'.
'ale'+'rts'='ALERTS'
(Other definitions for alerts that I've seen before include "Alarms, warning signals" , "Warns, draws attention to" , "danger-signals" , "Draws attention to, a danger perhaps" , "Puts on guard, forewarns" .)