Note start of month has new moon, occasionally (4)
I believe the answer is:
memo
'note' is the definition.
(I know that memo is a type of note)
'start of month has new moon occasionally' is the wordplay.
'start of' suggests taking the first letters.
'has' says to put letters next to each other.
'occasionally' indicates alternate letters.
The initial letter of 'month' is 'm'.
'new'+'moon'='newmoon'
The alternating letters of 'newmoon' are 'emo'.
'm'+'emo'='MEMO'
(Other definitions for memo that I've seen before include "diplomatic communication?" , "Reminder (abbr)" , "Written business message, in short" , "Brief note, usually between co-workers" , "Note written as a reminder" .)