The most common and most likely answer to this crossword clue is the 8 letter word FUSILIER
I've seen this crossword clue in Financial Times
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More clues leading to the result FUSILIER
Buddy explains!

The straight part
We know that "infantyman" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case infantyman = fusilier.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 FUSILIER mean?
FUSILIER noun- An infantryman armed with a form of flintlock musket
- A soldier in any of several regiments that once fought with such weapons
- A fish in family Caesionidae, related to snappers