The most common and most likely answer to this crossword clue is the 5 plus 7 letter word CAROL SERVICE
I've seen this crossword clue in The Daily Telegraph
C
A
R
O
L
S
E
R
V
I
C
E
Buddy explains!
The straight part
We know that "seasonal act of worship" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seasonal act of worship = carol service.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.