Crossword Genius

Decline appointment, keeping outside but dropping by (11)

Ross

I believe the answer is:

deteriorate

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

'decline' is the definition.
(deteriorating is a kind of declining)

'appointment keeping outside but dropping by' is the wordplay.
I cannot quite understand how this works, but
'by' could be 'at' and 'at' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for deteriorate that I've seen before include "Go downhill" , "Get worse" , "Become steadily worse" , "Grow worse" , "decline" .)

I've seen this clue in The Times.
Want a hint initially instead of a full solution? Install my app