The most common and most likely answer to this crossword clue is the 5 plus 3 letter word ALTAR BOY
I've seen this crossword clue in The Daily Telegraph
A
L
T
A
R
B
O
Y
More clues leading to the result ALTAR BOY
Buddy explains!
The straight part
We know that "He’s active in the masses observed by the ministry, sometimes getting cross" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case he’s active in the masses observed by the ministry, sometimes getting cross = altar boy.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.