All those attending, force to leave (7)
I believe the answer is:
turnout
'leave' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'all those attending force' is the wordplay.
I cannot really see how this works, but
'force' could be 'rn' (abbreviation for Royal Navy) and 'rn' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for turnout that I've seen before include "Number of people who come to a function, or their dress" , "Numbers attending an event" , "Eventuate; dismiss" , "Attendance figure" , "The crowd appearing at an event" .)