Misses one most, being crazy about (5)
I believe the answer is:
Here is my best explanation:
'misses' is the definition.
('omit' can be a synonym of 'miss')
'one most being crazy about' is the wordplay.
'one' becomes 'i' (Roman numeral).
'being crazy' indicates anagramming the letters.
'about' indicates putting letters inside.
'most' is an anagram of 'omts'.
'i' going into 'omts' is 'OMITS'.
(Other definitions for omits that I've seen before include "misses out", "Leaves out or neglects", "skips", "Forgets" and "overlooks".)