School backing measure to stock soft type of drink (8)

I believe the answer is:

schnapps

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'type of drink' is the definition.

'school backing measure to stock soft' is the wordplay.
'school' becomes 'sch' (abbreviation).
'backing measure' becomes 'nas' (I can't explain this - if you can you should believe this answer much more).
'to stock' means one lot of letters goes inside another (some letters contain or stock others).
'soft' becomes 'pp' (musical abbreviation for pianissimo).
'nas' going around 'pp' is 'napps'.
'sch'+'napps'='SCHNAPPS'

Can you help me to learn more?

(Other definitions for schnapps that I've seen before include "Holland gin" , "Strong gin-like drink" , "Booze" , "Strong spirit drink" , "Various strong spirits drunk in N Europe, consonant heavy" .)

I've seen this clue in The Telegraph.

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