The most common and most likely answer to this crossword clue is the 13 letter word EFFECTIVENESS
I've seen this crossword clue in The Daily Telegraph
E
F
F
E
C
T
I
V
E
N
E
S
S
Buddy explains!

The straight part
We know that "potency" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case potency = effectiveness.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 EFFECTIVENESS mean?
EFFECTIVENESS noun- The property of being effective, of achieving results.
- The capacity or potential for achieving results.
- The degree to which something achieves results.