Gathering for church service with uncrowned head of state (7)

I believe the answer is:

massing

I'm a little stuck... Click here to teach me more about this clue!

'gathering' is the definition.
The answer and definition can be both to do with motion as well as being present participle verbs.
Perhaps they are linked in a way I don't understand?

'church service with uncrowned head of state' is the wordplay.
'church service' becomes 'mass' (I've seen this before).
'with uncrowned head of state' becomes 'ing' (I am not sure about this - if you are sure you should believe this answer much more).
'mass'+'ing'='MASSING'

'for' is the link.

Can you help me to learn more?

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.