Greek god is a companion at heart (3)
I believe the answer is:
pan
'greek god' is the definition.
(Pan is an example)
'is a companion at heart' is the wordplay.
'at heart' says the answer is hidden in the clue (letters hidden in the heart of the clue).
'PAN' can be found hidden inside 'is companion'.
(Other definitions for pan that I've seen before include "Greek god of flocks and heards" , "Severely criticise; god" , "Greek woodland god" , "Wash gravel" , "Slate - cooking vessel" .)