The most common and most likely answer to this crossword clue is the 4 plus 2 letter word FILL IN
I've seen this crossword clue in Independent
F
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L
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N
More clues leading to the result FILL IN
Buddy explains!

The straight part
We know that "Occupy one’s spare time/punch" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case occupy one’s spare time/punch = fill in.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 FILL IN mean?
FILL IN noun- A temporary replacement for another, especially at a job.
- A substitution.
- An intermediate result that must be stored temporarily during the course of a sparse matrix computation.
- A question or puzzle in which one is expected to fill in a missing part of something.
- A musical embellishment (usually percussion) that is added to connect musical phrases.
- Something added to fill a gap.
- Something added to increase the size of something; padding or filler.
- A product category that is used to complete a range or variety of a product line.