Darling, promise of marriage's a little late (4)
I believe the answer is:
idol
'darling' is the definition.
(I've seen this before)
'promise of marriage's a little late' is the wordplay.
'promise of marriage's' becomes 'ido' (I can't justify this - if you can you should believe this answer much more).
'a little' says to take the initial letters.
The initial letter of 'late' is 'l'.
'ido'+'l'='IDOL'
Can you help me to learn more?
(Other definitions for idol that I've seen before include "Graven image or worshipped person" , "Admired figure" , "False god" , "A matinee -- is a handsome actor" , "loved one" .)