The most common and most likely answer to this crossword clue is the 7 letter word RODENTS
R
O
D
E
N
T
S
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The straight part
We know that "They gnaw" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they gnaw = rodents.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 RODENTS mean?
RODENTS noun- A mammal of the order Rodentia, characterized by long incisors that grow continuously and are worn down by gnawing.
- (bulletin board system slang, leet) A person lacking in maturity, social skills, technical competence or intelligence; lamer.