The most common and most likely answer to this crossword clue is the 6 letter word SIMOOM
S
I
M
O
O
M
Buddy explains!
The straight part
We know that "that’s a blow" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s a blow = simoom.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 SIMOOM mean?
SIMOOM noun- A hot, dry, suffocating, dust-laden wind of the desert, particularly of Arabia, Syria, and neighboring countries, generated by the extreme heat of the parched deserts or sandy plains.