The heart of the country (7)
I believe the answer is:
midland
'the' is the definition.
I can't judge whether this definition defines the answer.
'heart of the country' is the wordplay.
'heart of' becomes 'mid' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the country' becomes 'land' (synonyms).
'mid'+'land'='MIDLAND'
Can you help me to learn more?
(Other definitions for midland that I've seen before include "In a country's central part" , "In the centre of a country" , "Birmingham, perhaps" .)