The most common and most likely answer to this crossword clue is the 5 plus 7 letter word ACTOR MANAGER
I've seen this crossword clue in The Daily Telegraph
A
C
T
O
R
M
A
N
A
G
E
R
Buddy explains!
The straight part
We know that "One taking the part of the boss" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one taking the part of the boss = actor manager.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.