Stuff one has to leave in place (3,3)
I believe the answer is:
pig out
'stuff' is the definition.
('pig out' can be a synonym of 'stuff')
'one has to leave in place' is the wordplay.
'one' becomes 'i' (Roman numeral).
'has to leave' becomes 'go' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'place' becomes 'put' ('put' can be a synonym of 'place').
'i'+'go'='igo'
'igo' placed inside 'put' is 'PIG OUT'.
Can you help me to learn more?
(Other definitions for pig out that I've seen before include "go on a binge" , "overindulge" , "To eat a lot may" , "Eat too much food" , "gorge" .)