Celebration third at sea, within cove (8)
I believe the answer is:
birthday
'celebration' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'third at sea within cove' is the wordplay.
'at sea' is an anagram indicator.
'within' means one lot of letters goes inside another.
'cove' becomes 'bay' (both can mean a coastal inlet).
'third' anagrammed gives 'irthd'.
'irthd' placed within 'bay' is 'BIRTHDAY'.
Can you help me to learn more?
(Other definitions for birthday that I've seen before include "When presents are expected" , "Hardy bit (anag)" , "Natal anniversary" , "It comes around once a year" , "when one was delivered" .)