Mate carrying one bucket (4)
I believe the answer is:
pail
'bucket' is the definition.
('pail' can be a synonym of 'bucket')
'mate carrying one' is the wordplay.
'mate' becomes 'pal' (I've seen this before).
'carrying' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'pal' placed around 'i' is 'PAIL'.
(Other definitions for pail that I've seen before include "Milkmaid's bucket?" , "Open-topped container" , "A bucket as for water" , "it may go down well" , "A bucket - the kind Jack and Jill used" .)