Sacred tree loses heart (4)
I believe the answer is:
holy
'sacred' is the definition.
(similar in meaning)
'tree loses heart' is the wordplay.
'tree' becomes 'holly' (holly is a kind of tree).
'loses heart' says to hollow out the word (remove centre letters).
'holly' with its centre removed is 'HOLY'.
(Other definitions for holy that I've seen before include "Sacred" , "Charlemagne was the first" , "perfect, morally and spiritually" , "Dedicated to God" , "Divine" .)