A trusty heart can be a bad thing! (4)
I believe the answer is:
rust
'a bad thing' is the definition.
The definition and answer can be both states of affairs as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'a trusty heart' is the wordplay.
'heart' means to look at the middle letters.
The central letters of 'trusty' are 'RUST'.
'can be' acts as a link.
Can you help me to learn more?
(Other definitions for rust that I've seen before include "Red or brown oxide coating on iron or steel" , "Corrode" , "Fungal disease of plants" , "Fungal plant disease" , "Reddish oxide coating on iron" .)