Cryptic crossword clue
Determinedly set, that guy is going to be not disheartened 4-4

The most common and most likely answer to this crossword clue is the 4 plus 4 letter word HELL BENT

I've seen this crossword clue in The Sunday Telegraph
H
E
L
L
B
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T


Buddy explains!

The straight part
We know that "Determinedly set" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case determinedly set = hell bent.
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 HELL BENT mean?

HELL BENT adjective
  1. Recklessly determined to do or achieve something

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