The most common and most likely answer to this crossword clue is the 7 letter word MASTERY
I've seen this crossword clue in The Guardian
M
A
S
T
E
R
Y
More clues leading to the result MASTERY
Buddy explains!

The straight part
We know that "Proficiency" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case proficiency = mastery.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 MASTERY mean?
MASTERY noun- The position or authority of a master; dominion; command; supremacy; superiority.
- Superiority in war or competition; victory; triumph; preeminence.
- Contest for superiority.
- A masterly operation; a feat.
- The philosopher's stone.
- The act or process of mastering; the state of having mastered; expertise.