Big excursion round old desert (4)

I believe the answer is:

gobi

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

'desert' is the definition.
(Gobi is an example)

'big excursion round old' is the wordplay.
'big excursion' becomes 'gob' (I can't explain this - if you can you should give a lot more credence to this answer).
'round' means to look at the middle letters.
'old' becomes 'anile' ('anile' can be similar in meaning to 'old').
The middle letter of 'anile' is 'i'.
'gob'+'i'='GOBI'

Can you help me to learn more?

(Other definitions for gobi that I've seen before include "Desert, mostly in Mongolia" , "Plain" , "Asian desert, Chinese name Shamo" , "Desert in China/Mongolia; cauliflower (Ind.)" , "Mongolian expanse" .)

I've seen this clue in The Mirror.

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