A number being sentimental about love regularly (6)
I believe the answer is:
twelve
'a number' is the definition.
(twelve is a kind of number)
'sentimental about love regularly' is the wordplay.
'sentimental' becomes 'twee' (I've seen this before).
'about' means one lot of letters goes inside another.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'love' are 'lv'.
'twee' going around 'lv' is 'TWELVE'.
'being' is the link.
(Other definitions for twelve that I've seen before include "Apostles and members of the jury" , "[MIDNIGHT], perhaps" , "Midnight or noon" , "One under a baker's dozen" , "The midnight hour" .)