It's most expensive darling (7)
I believe the answer is:
dearest
I believe this clue is a double definition.
'it's most expensive' is the first definition.
('dear' can be similar in meaning to 'expensive')
'darling' is the second definition.
(both are terms of endearment)
(Other definitions for dearest that I've seen before include "Darling" , "Most precious or expensive" , "most excessive" , "The beloved one is most costly" , "Most loved or expensive" .)