At sea, left specialist a sign (7)
I believe the answer is:
portent
'a sign' is the definition.
(a portent is a sign of something)
'at sea left specialist' is the wordplay.
'at' is a charade indicator (letters next to each other).
'sea left' becomes 'port' (left side of a ship - opposite of starboard. I am not sure about the 'sea' bit.).
'specialist' becomes 'ent' (Ear, Nose and Throat specialist).
'port' put next to 'ent' is 'PORTENT'.
Can you help me to learn more?
(Other definitions for portent that I've seen before include "Evil omen" , "Sign of thing to come" , "Prophetic sign" , "Augury" , "Sign, omen" .)