The most common and most likely answer to this crossword clue is the 6 letter word LETHAL
I've seen this crossword clue in The Times
L
E
T
H
A
L
More clues leading to the result LETHAL
- Allow prince to be very nasty indeed
- Deadly time to stab the French prince
- Allowed Shakespeare's prince to be very destructive
- All the ground is deadly!
- Part of shibboleth — a little deadly
- All the changes of a deadly nature
- Not all fillet halibut with devastating ability
- Tablet halved: centre is deadly
- Leased 2001 computer – fatal!
- Causing death is allowed by 2001 computer
Buddy explains!

The straight part
We know that "deadly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deadly = lethal.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 LETHAL mean?
LETHAL noun- Any weapon that causes death.
- An allele that causes the death of the organism that carries it.
- Deadly; mortal; fatal.
- One of the higher alcohols of the paraffine series obtained from spermaceti as a white crystalline solid.