The most common and most likely answer to this crossword clue is the 4 letter word BONY
I've seen this crossword clue in The Times
B
O
N
Y
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 = bony.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 BONY mean?
BONY adjective- Resembling, having the appearance or consistence of, or relating to bone; osseous.
- Full of bones
- With little flesh; skinny, thin
- Having prominent bones