The most common and most likely answer to this crossword clue is the 6 letter word GUNNER
I've seen this crossword clue in Financial Times
G
U
N
N
E
R
Buddy explains!

The straight part
We know that "A cocktail/a soldie" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a cocktail/a soldie = gunner.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 GUNNER mean?
GUNNER noun- Artillery soldier, or such who holds private rank. Abbreviated Gnr.
- A person who operates a gun.
- An excessive go-getter; one exhibiting over-ambition.
- A player on the kicking team whose primary job is to tackle the kickoff returner or punt returner.
- A fan of the Arsenal Football Club.
- The great northern diver or loon.
- The sea bream, especially Pagellus bogaraveo (blackspot sea bream)
- A player who can reliably shoot baskets.
- (Cebu) The person designated to pour drinks in a drinking session.