Dairy in South Minehead returned countless numbers (7)

I believe the answer is:

myriads

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

'countless numbers' is the definition.
'myriads' can be an answer for 'numbers' (myriad is a kind of number). I am not certain of the 'countless' bit.

'dairy in south minehead returned' is the wordplay.
'in' indicates putting letters inside.
'south' becomes 'S' (synonyms).
'minehead' becomes 'm' (head letter of 'mine').
'returned' shows that the letters should be reversed in order.
's'+'m'='sm'
'dairy' inserted inside 'sm' is 'sdairym'.
'sdairym' reversed gives 'MYRIADS'.

Can you help me to learn more?

(Another definition for myriads that I've seen is " huge numbers".)

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

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