The most common and most likely answer to this crossword clue is the 12 letter word OCCASIONALLY
I've seen this crossword clue in The Daily Telegraph
O
C
C
A
S
I
O
N
A
L
L
Y
More clues leading to the result OCCASIONALLY
Buddy explains!

The straight part
We know that "Now and then" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case now and then = occasionally.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.
What does OCCASIONALLY mean?
OCCASIONALLY adverb- On the occasion of something else happening; incidentally, by the way.
- From time to time; sometimes; at relatively infrequent intervals.
- By chance; accidentally.