The most common and most likely answer to this crossword clue is the 6 letter word JUNKIE
I've seen this crossword clue in Financial Times
J
U
N
K
I
E
More clues leading to the result JUNKIE
Buddy explains!

The straight part
We know that "User" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case user = junkie.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 JUNKIE mean?
JUNKIE noun- A narcotics addict, especially a heroin user.
- (by extension) An enthusiast of something.