Lack of knowledge of racing — one at sea (9)
I believe the answer is:
ignorance
'lack of knowledge' is the definition.
(I've seen this before)
'racing one at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'racing'+'one'='racingone'
'racingone' is an anagram of 'IGNORANCE'.
'of' acts as a link.
(Other definitions for ignorance that I've seen before include "Lack of education" , "Unintelligence" , "'Where . . . . . . . . . is bliss, 'tis folly to be wise'" , "lack of awareness" , "Not knowing" .)