The most common and most likely answer to this crossword clue is the 9 letter word SALTPETRE
I've seen this crossword clue in The Times
S
A
L
T
P
E
T
R
E
More clues leading to the result SALTPETRE
- Explosive component to season: about to go past favourite
- Later step developed a constituent of gunpowder
- Old sailor preferred dealing with explosive ingredient
- Liveliness with power to be for Sarkozy an explosive component
- An ingredient of gunpowder later step developed
- Ingredient of explosive soldier favoured in revolutionary times
- Sailor about to collect special gunpowder ingredient
- Chemical alert — pet’s ill
- Sailor’s pique concerning compound
- Sailor with bad temper shells out money to get gunpowder
Buddy explains!

The straight part
We know that "a constituent of gunpowder" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a constituent of gunpowder = saltpetre.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 SALTPETRE mean?
SALTPETRE noun- Potassium nitrate.
- Sodium nitrate.