Cryptic crossword clue
Uncompromising, aggressive advertising that’s hard to stop 9

The most common and most likely answer to this crossword clue is the 9 letter word HARDSHELL

I've seen this crossword clue in Financial Times
H
A
R
D
S
H
E
L
L

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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 HARDSHELL mean?

HARDSHELL adjective
  1. Having a tough outer shell
  2. Rigid or uncompromising in principles, especially in religion or politics.

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