The most common and most likely answer to this crossword clue is the 6 letter word FINIAL
I've seen this crossword clue in The Guardian
F
I
N
I
A
L
More clues leading to the result FINIAL
- Decorative ornament: last one placed inside
- Topping decoration I installed in terminal
- Nail destroyed post providing backing for spire’s ornament
- One enters concluding term in architecture
- Ornamental piece I put in last
- Last word of feature in French, setting up what follows so one gets crowning 18
- Compiler’s in last ornament
- Dying to accept one’s decoration
- Decoration for which I must be put in last
- Prominent ornament I put in last
Buddy explains!

The straight part
We know that "spire’s ornament" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case spire’s ornament = finial.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 FINIAL mean?
FINIAL noun- The knot or bunch of foliage, or foliated ornament, that forms the upper extremity of a pinnacle in Gothic architecture.
- Any decorative fitting at the peak of a gable, or on the top of a flagpole, fence post or staircase newel post.