Element identified by sharp-witted cook at sea (11)
I believe the answer is:
quicksilver
'element' is the definition.
(quicksilver is an old name for mercury)
'sharp-witted cook at sea' is the wordplay.
'sharp-witted' becomes 'quick' (I've seen this in another clue).
'cook at sea' becomes 'silver' (Long John Silver in the novel Treasure Island).
'quick'+'silver'='QUICKSILVER'
'identified by' acts as a link.
(Other definitions for quicksilver that I've seen before include "so rapid" , "Mercury - volatile" , "Liquid mercury" , "Element of dentistry" .)