Vessel in which one takes a chum out (4)
I believe the answer is:
pail
'vessel' is the definition.
(pail is a kind of vessel)
'one takes a chum out' is the wordplay.
'one takes' becomes 'i' (I can't explain this - if you can you should give a lot more credence to this answer).
'a chum' becomes 'pal' ('pal' can be a synonym of 'chum').
'out' means one lot of letters goes inside another.
'i' put into 'pal' is 'PAIL'.
'in which' acts as a link.
Can you help me to learn more?
(Other definitions for pail that I've seen before include "Vessel for carrying liquids" , "A bucket as for water" , "A bucket - the kind Jack and Jill used" , "Cylindrical vessel" , "Kind of bucket - ask Jack and Jill" .)