At sea no rating is lacking in knowledge (8)
I believe the answer is:
ignorant
'lacking in knowledge' is the definition.
(I've seen this before)
'at sea no rating' is the wordplay.
'at sea' indicates anagramming the letters.
'no'+'rating'='norating'
'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" .)