Couple have short letter for homework (4)

I believe the answer is:

prep

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'homework' is the definition.
('prep' can be a synonym of 'homework')

'couple have short letter' is the wordplay.
'couple' becomes 'pr'.
'have' says to put letters next to each other.
'short letter' becomes 'ep' (I can't explain this - if you can you should give a lot more credence to this answer).
'pr'+'ep'='PREP'

'for' acts as a link.

Can you help me to learn more?

(Other definitions for prep that I've seen before include "warm-up race" , "Homework (abbr.)" , "School work done outside lessons" , "Homework (abbrev.)" .)

I've seen this clue in The Telegraph.

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