Teacher with only one shoe? (7)

I believe the answer is:

trainer

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

This clue could be a double definition.

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

'only one shoe?' is the second definition.
'trainer' can be an answer for 'shoe?' (I've seen this before). I am not certain of the 'only one' bit.

'with' is the link.

Can you help me to learn more?

(Other definitions for trainer that I've seen before include "Comfortable shoe" , "Soft shoe, small aircraft" , "Casual shoe" , "teacher" , "Casual 11 ac" .)

I've seen this clue in the Evening Standard.

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