Middle Earth is back to front (5)
I believe the answer is:
heart
'middle' is the definition.
(synonyms)
'earth is back to front' is the wordplay.
'earth' can be anagrammed to 'HEART'.
But, I am not sure how this is indicated.
Can you help me to learn more?
(Other definitions for heart that I've seen before include "It's in the chest" , "Nucleus" , "the name of a prison in Midlothian!" , "Vital part" , "Middle" .)