The most common and most likely answer to this crossword clue is the 4 plus 6 letter word PAST MASTER
I've seen this crossword clue in The Daily Telegraph
P
A
S
T
M
A
S
T
E
R
More clues leading to the result PAST MASTER
- Expert's way accepted by Pamela and Daisy
- Expert history teacher
- Expert history teacher?
- Stamp designed by flower expert
- Expert rates stamp poorly
- Old hand gets tangled stems apart
- Male history teacher?
- History teacher is accomplished expert
- Expert from whom one can no longer learn anything
- Expert finds a trace of methylhexaneamine in sample after dope’s shown up
Buddy explains!

The straight part
We know that "Male history teacher" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case male history teacher = past master.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 PAST MASTER mean?
PAST MASTER noun- Someone who has previously been a master at a given lodge.
- One who has an extremely high level of ability or knowledge within an area of achievement or expertise.