Sacred form of trees (6)
I believe the answer is:
cedars
'trees' is the definition.
(cedar is a kind of tree)
'sacred form' is the wordplay.
'form' is an anagram indicator.
'sacred' is an anagram of 'CEDARS'.
'of' acts as a link.
(Other definitions for cedars that I've seen before include "Sacred (anag) -- trees" , "(Lebanese?) trees" , "Strangely sacred trees" , "Fragrant trees of the Lebanon" , "Aromatic trees" .)