Watched depart (4,3)

I believe the answer is:

seen off

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

'watched depart' is the definition.
I know nothing about this answer so I can't judge whether this works.

'watched depart' is the wordplay.
'watched' becomes 'seen' (I've seen this before).
'depart' becomes 'off' (I can't explain this - if you can you should give a lot more credence to this answer).
'seen'+'off'='SEEN OFF'

Can you help me to learn more?

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app