Mark loses heart — an unhealthy condition (4)
I believe the answer is:
coma
'an unhealthy condition' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe there's an association between them I don't understand?
'mark loses heart' is the wordplay.
'mark' becomes 'comma' (punctuation mark).
'loses heart' suggests removing the centre.
'comma' with its middle removed is 'COMA'.
Can you help me to learn more?
(Other definitions for coma that I've seen before include "long sleep" , "Unconsciousness from which person cannot be aroused" , "State of unconsciousness caused by illness" , "Long period of unconsciousness" , "State of insensibility" .)