Sweetheart low with start of new period after wedding (9)
I believe the answer is:
honeymoon
'period after wedding' is the definition.
(a married couple's holiday after a wedding)
'sweetheart low with start of new' is the wordplay.
'sweetheart' becomes 'honey' (I've seen this in another clue).
'low' becomes 'moo' (both can mean to make the noise of a cow).
'with' means one lot of letters go next to another.
'start of' indicates taking the first letters.
The first letter of 'new' is 'n'.
'honey'+'moo'+'n'='HONEYMOON'
(Other definitions for honeymoon that I've seen before include "Holiday of a newly married couple" , "romantic break" , "Newlyweds' holiday" , "Early period of goodwill" , "Holiday following wedding" .)