The most common and most likely answer to this crossword clue is the 6 letter word NULLAH
N
U
L
L
A
H
Buddy explains!
The straight part
We know that "blocking around drain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case blocking around drain = nullah.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 NULLAH mean?
NULLAH noun- A stream-bed, ravine, or other watercourse; a drain for rain or floodwater.