At sea no rating is lacking in knowledge (8)
I believe the answer is:
'lacking in knowledge' is the definition.
(I've seen this before)
'at sea no rating' is the wordplay.
'at sea' indicates anagramming the letters.
'norating' anagrammed gives 'IGNORANT'.
'is' is the link.
(Other definitions for ignorant that I've seen before include "Unaware, uninformed" , "Illiterate - unaware" , "Unlearned, lacking knowledge" , "Lacking knowledge or lacking manners" , "Lacking knowledge or awareness" .)