Warning left to ring Prime Minister's number (7)
I believe the answer is:
portent
'warning' is the definition.
(I've seen this before)
'left to ring prime minister's number' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'to ring' indicates putting letters inside.
'prime minister's number' becomes 'ten' (the prime minister lives in No 10 Downing Street).
'port' going around 'ten' is 'PORTENT'.
(Other definitions for portent that I've seen before include "Sign or warning of an event likely to happen" , "Prophetic sign" , "Sign of thing to come" , "Sign, omen" , "Warning of momentous event" .)