In which to pack one's trousers? (4)
I believe the answer is:
bags
'one's trousers?' is the definition.
'bags' can be an answer for 'trousers?' (informal term for trousers, Oxford bags). I am unsure of the 'one's' bit.
'in which to pack' is the wordplay.
'in which' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'pack' becomes 'bag' (bagging is a kind of packing).
's' after 'bag' is 'BAGS'.
Can you help me to learn more?
(Other definitions for bags that I've seen before include "Criticises" , "Successfully secures" , "Childishly claims" , "Lots of" , "Flexible containers, of paper say" .)