The most common and most likely answer to this crossword clue is the 12 plus 6 letter word FAIR-WEATHER FRIEND
I've seen this crossword clue in The Times
F
A
I
R
-
W
E
A
T
H
E
R
F
R
I
E
N
D
Buddy explains!
The straight part
We know that "an unreliable acquaintance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an unreliable acquaintance = fair-weather friend.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.