The most common and most likely answer to this crossword clue is the 3 plus 9 letter word OLD MACDONALD
I've seen this crossword clue in The Guardian
O
L
D
M
A
C
D
O
N
A
L
D
Buddy explains!
The straight part
We know that "involved in his work" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case involved in his work = old macdonald.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.