The most common and most likely answer to this crossword clue is the 6 letter word WYVERN
W
Y
V
E
R
N
More clues leading to the result WYVERN
- Case of Welshman bagging very wild dragon
- Heraldic beast with very strange name
- How come sci-fi writer's spoken of monster?
- Wherefore, reportedly, sci-fi writer briefly identifies winged dragon
- Winged beast very excitedly going in two directions
- Externally, woman — internally, very furious dragon
- Heraldic device with very strange name
- Heraldic beast getting first of wizards terribly nervy
- Points out very different 1950s Vauxhall model
- Dragon, mostly green, captured by Welshman
Buddy explains!

The straight part
We know that "monster" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case monster = wyvern.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 WYVERN mean?
WYVERN noun- A draconian creature possessing wings, only two legs and usually a barbed tail.