Communist’s sharp warning (3,5)
I believe the answer is:
red alert
'warning' is the definition.
(I've seen this before)
'communist's sharp' is the wordplay.
'communist' becomes 'red' (red is a kind of communist).
'sharp' becomes 'alert' (synonyms).
'red'+'alert'='RED ALERT'
(Other definitions for red alert that I've seen before include "State of readiness for imminent crisis" , "High crisis level" , "Urgent warning" , "Serious warning" , "Warning signal" .)