Seasonal entertainment used to be a sea voyage (7)
I believe the answer is:
wassail
'seasonal entertainment' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'used to be a sea voyage' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'a sea voyage' becomes 'sail' (sail is a kind of voyage. I am not sure about the 'sea' bit.).
'was'+'sail'='WASSAIL'
Can you help me to learn more?
(Other definitions for wassail that I've seen before include "celebration" , "Go carol-singing" , "Carouse" , "festive drink" , "Sing Christmas carols" .)