Left net out as a sign (7)
I believe the answer is:
portent
'a sign' is the definition.
(portent is a kind of sign)
'left net out' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'net' is an anagram of 'ent'.
'port'+'ent'='PORTENT'
'as' acts as a link.
(Other definitions for portent that I've seen before include "Sign, omen" , "Foreboding" , "Prophetic sign" , "Sign or warning of an event likely to happen" , "Sign of thing to come" .)