Definition: Used as an affectionate form of address to a beloved person.
The term "darling" is a part of the English-speaking culture that there is no direct word-for-word translation of this nor usage of this in ASL.
But, there are some ways of signing ASL equivalent to this concept such as waving ILY, "5" handshape on the chest, and such in contexts.
Again, this meaning may change in different contexts. It may be something along the way as DARLING, BELOVED.