It's soft, that is, with a crust (3)
I believe the answer is:
pie
'a crust' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'it's soft that is' is the wordplay.
'it's soft' becomes 'p' (musical abbreviation for piano, 'soft').
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'p'+'ie'='PIE'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for pie that I've seen before include "Pecan or blueberry dish?" , "In the sky, this dish is impractical" , "Covered tart" , "Anna's constituent" , "steak might go in this" .)