Heart of naturalist is in the mountains! (4)
'the mountains' is the definition.
(the Ural Mountains)
'heart of naturalist is' is the wordplay.
'heart of' indicates a hidden word.
'URAL' is hidden in the letters of 'naturalist is'.
'in' acts as a link.
(Other definitions for ural that I've seen before include "Russian river flowing into the Caspian Sea", "Mountains in Russia", "River of central Russia", "Russian river giving name to mountains", "Mountain range between Europe and Asia".)