Tree initially as charming as collection in arboretum (6)
I believe the answer is:
acacia
'tree' is the definition.
(acacia is a kind of tree)
'initially as charming as collection in arboretum' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'as charming as collection in arboretum' is 'ACACIA'.
(Other definitions for acacia that I've seen before include "Type of shrub or tree" , "Flowering tree or shrub" , "Spiny tree or shrub" , "Australian tree" , "Gum-yielding tree ir shrub" .)