Heralds missing first new trees (6)
I believe the answer is:
alders
'trees' is the definition.
(alder is a kind of tree)
'heralds missing first new' is the wordplay.
'missing first' means to remove the first letter.
'new' indicates an anagram.
'heralds' with its initial letter taken off is 'eralds'.
'eralds' is an anagram of 'ALDERS'.
(Other definitions for alders that I've seen before include "Trees for red Sal" , "Trees of the birch family" , "Trees with toothed leaves" , "Al's red trees, strangely" , "Catkin-bearing trees" .)