Lady who is crazy to get up before sweetheart (4)
I believe the answer is:
dame
'lady' is the definition.
('dame' can be a synonym of 'lady')
'is crazy to get up before sweetheart' is the wordplay.
'is crazy' becomes 'mad' ('mad' can be a synonym of 'crazy'**).
'to get up' says the letters should be written in reverse.
'before' is a charade indicator (letters next to each other).
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'mad' reversed gives 'dam'.
'dam'+'e'='DAME'
'who' is the link.
(Other definitions for dame that I've seen before include "Woman" , "Female equivalent of a knight" , "British title for a lady" , "Title given to women" , "Pantomime character" .)