The most common and most likely answer to this crossword clue is the 7 letter word RECTORS
I've seen this crossword clue in The Daily Telegraph
R
E
C
T
O
R
S
Buddy explains!
The straight part
We know that "mass conductors" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mass conductors = rectors.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 RECTORS mean?
RECTORS noun- In the Anglican Church, a cleric in charge of a parish and who owns the tithes of it.
- In the Roman Catholic Church, a cleric with managerial as well as spiritual responsibility for a church or other institution.
- A priest or bishop who is in charge of a parish or in an administrative leadership position in a theological seminary or academy.
- In a Protestant church, a pastor in charge of a church with administrative and pastoral leadership combined.
- A headmaster in various educational institutions, e.g. a university.