Waterproof garment, medium type, mostly (7)

I believe the answer is:

oilskin

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'waterproof' is the definition.
(I know that oilskin is a type of Mackintosh)

'garment medium type mostly' is the wordplay.
'garment medium' becomes 'oils' (I can't explain this - if you can you should believe this answer much more).
'type' becomes 'kind' (I've seen this before).
'mostly' means to remove the last letter (most but not all of the word).
'kind' with its final letter taken off is 'kin'.
'oils'+'kin'='OILSKIN'

Can you help me to learn more?

(Other definitions for oilskin that I've seen before include "Raincoat" , "Waterproof cloth for rain-gear" , "Heavy-duty waterproof" , "foul weather item" , "Heavy waterproofed cloth" .)

I've seen this clue in The Times.

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