The most common and most likely answer to this crossword clue is the 12 letter word SUPPLEMENTAL
I've seen this crossword clue in The Times
S
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More clues leading to the result SUPPLEMENTAL
Buddy explains!

The straight part
We know that "Spare" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case spare = supplemental.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 SUPPLEMENTAL mean?
SUPPLEMENTAL noun- Something that supplements or adds to.
- A requisition or article of legislation that provides additional funding for a program.
- Acting to supplement.
- Appending.
- Lacking a regular schedule (as in a supplemental airline).