Bitter winter frost ends, sharp (5)
I believe the answer is:
alert
'sharp' is the definition.
('alert' can be a synonym of 'sharp')
'bitter winter frost ends' is the wordplay.
'bitter' becomes 'ale' ('ale' can be a synonym of 'bitter').
'ends' says to take the final letters.
The final letters of 'winter frost' are 'rt'.
'ale'+'rt'='ALERT'
(Other definitions for alert that I've seen before include "Watchful, quick-thinking" , "Wide-awake and watchful" , "Lively" , "Wide awake and watchful" , "On one's toes" .)