Quick expulsion of unknown European, first seen at sea (6)
I believe the answer is:
sneeze
'quick expulsion' is the definition.
'unknown european first seen at sea' is the wordplay.
'unknown' becomes 'z' (algebra).
'european first' becomes 'e' (1st letter of 'european').
'at sea' is an anagram indicator.
'z'+'e'+'seen'='zeseen'
'zeseen' is an anagram of 'SNEEZE'.
'of' is the link.
(Other definitions for sneeze that I've seen before include "symptom of flu" , "Response to nasal irritation" , "Audible and uncontrolled expulsion of air through nose and mouth" , "Symptom of cold" , "Sign of the common cold" .)