The most common and most likely answer to this crossword clue is the 6 letter word MUSCID
M
U
S
C
I
D
Buddy explains!
The straight part
We know that "Fly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fly = muscid.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 MUSCID mean?
MUSCID noun- Any fly of the family Muscidae of insects.
- Pertaining to or related to the Muscidae family of insects.