The most common and most likely answer to this crossword clue is the 4 plus 5 letter word KING COBRA
I've seen this crossword clue in Independent
K
I
N
G
C
O
B
R
A
More clues leading to the result KING COBRA
- Asian giant oak's last big acorn cracked
- Family company in GB meeting artist, a slithery creature
- Hooded Asian ruler leading troops on horse
- Venomous type, man attracting firm support
- Hooded killer from Royal Horse Artillery
- Venomous sort caught wearing half-torn item of hosiery over lingerie
- Venomous type? Man has company support
- Reptile family’s finally flourishing with firm support
- Big rack on awfully dangerous creature
- Put out cigar and bonk Serpent
Buddy explains!

The straight part
We know that "Serpent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case serpent = king cobra.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.