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.
Perhaps they are linked in a way I don't understand?
'a trusty heart' is the wordplay.
'heart' indicates the central letters.
The central letters of 'trusty' are 'RUST'.
'can be' is the 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" .)