Not initially completely evergreen (5)
I believe the answer is:
holly
'evergreen' is the definition.
(I have seen 'Prickly evergreen ' mean 'holly' so perhaps 'evergreen' could also mean 'holly')
'not initially completely' is the wordplay.
'not initially' means to remove the first letter.
'completely' becomes 'wholly' (synonyms).
'wholly' with its initial letter removed is 'HOLLY'.
(Other definitions for holly that I've seen before include "Prickly evergreen" , "Girl" , "Tree with prickles" , "Shrub with scarlet berries" , "... and ivy, Yuletide decoration" .)