The most common and most likely answer to this crossword clue is the 5 plus 3 plus 5 letter word FETCH AND CARRY
I've seen this crossword clue in The Guardian
F
E
T
C
H
A
N
D
C
A
R
R
Y
Buddy explains!
The straight part
We know that "how to act the part of a servant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case how to act the part of a servant = fetch and carry.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.