Leave out some heirloom I treasure (4)
I believe the answer is:
Here is my best explanation:
'leave out' is the definition.
('omit' can be a synonym of 'leave out')
'some heirloom i treasure' is the subsidiary indication.
'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 "Drop", "Fail to include", "Leave something out", "Leave out or undone", "Fail (to do something)", "Leave out or fail to do", "Leave out or exclude", "Leave out, do not include (4)", "Overlook" and "Skip".)