The most common and most likely answer to this crossword clue is the 10 letter word MICROFARAD
I've seen this crossword clue in The Guardian
M
I
C
R
O
F
A
R
A
D
Buddy explains!
The straight part
We know that "Measure of capacity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case measure of capacity = microfarad.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 MICROFARAD mean?
MICROFARAD noun- One millionth ( 10-6 ) of a farad, abbreviated as µF.