The most common and most likely answer to this crossword clue is the 4 plus 1 plus 5 plus 2 letter word HAVE A CRUSH ON
I've seen this crossword clue in The Daily Telegraph
H
A
V
E
A
C
R
U
S
H
O
N
Buddy explains!
The straight part
We know that "Fancy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fancy = have a crush on.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.