Leave out some heirloom I treasure (4)
I believe the answer is:
omit
'leave out' is the definition.
('omit' can be a synonym of 'leave out')
'some heirloom i treasure' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'OMIT' can be found hidden inside 'heirloom i treasure'.
(Other definitions for omit that I've seen before include "Disregard" , "Fail to include" , "Leave something out" , "Leave out or undone" , "Fail (to do something)" .)