Shrub head of arboretum is able so to provide (8)
I believe the answer is:
acanthus
'shrub' is the definition.
(I've seen this before)
'head of arboretum is able so to provide' is the wordplay.
I cannot quite understand how this works, but
'able' could be 'can' and 'can' is present in the answer.
'so' could be 'thus' ('thus' can be a synonym of 'so') and 'thus' is present in the answer.
A single letter 'a' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for acanthus that I've seen before include "Prickly-leaved shrub" , "capital decoration" , "Carved ornament" , "Herbaceous shrub" , "Leaf, sometimes carved on furniture" .)