Finish sending first letter off (6)

I believe the answer is:

ending

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

'finish' is the definition.
(I know that finish can be written as ending)

'sending first letter off' is the wordplay.
'ENDING' is hidden in amongst the letters.
However, I am uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for ending that I've seen before include "Stopping" , "Terminating" , "Finale" , "Closing down" , "Close - conclusion" .)

I've seen this clue in the Sydney Morning Herald.

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