Mother briefly has ex-PM as teacher (9)

I believe the answer is:

maharishi

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'teacher' is the definition.
(I've seen this before)

'mother briefly has ex-pm' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'briefly' means to remove the last letter.
'ex pm' becomes 'rishi' (I can't justify this - if you can you should give a lot more credence to this answer).
'has' with its final letter taken off is 'ha'.
'ma'+'ha'+'rishi'='MAHARISHI'

'as' is the link.

Can you help me to learn more?

(Other definitions for maharishi that I've seen before include "Seasoned beef-pork sausage" , "Teacher of Hindu mysticism" , "Religious leader" , "Hindu spiritual leader" , "Religious teacher" .)

I've seen this clue in The Guardian.

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