It's the most expensive, darling (7)
I believe the answer is:
dearest
This clue is a double definition.
'it's the most expensive' is the first definition.
(I've seen this before)
'darling' is the second definition.
(both are terms of endearment)
(Other definitions for dearest that I've seen before include "Affectionate address" , "most delightful" , "Most loved or expensive" , "Most precious or expensive" , "most excessive" .)