The most common and most likely answer to this crossword clue is the 8 letter word PIRATING
I've seen this crossword clue in The Daily Telegraph
P
I
R
A
T
I
N
G
More clues leading to the result PIRATING
Buddy explains!

The straight part
We know that "Not if he’s doing this" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not if he’s doing this = pirating.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 PIRATING mean?
PIRATING verb- To appropriate by piracy, plunder at sea.
- (intellectual property) To create and/or sell an unauthorized copy of
- (intellectual property) To knowingly obtain an unauthorized copy of
- To engage in piracy.