Tree so great shelters the old woman (6)
I believe the answer is:
sumach
'tree' is the definition.
(I have seen 'a tree' mean 'sumach' so perhaps 'tree' could also mean 'sumach')
'so great shelters the old woman' is the wordplay.
'so great' becomes 'such'.
'shelters' indicates putting letters inside.
'the old woman' becomes 'ma' (both can mean someone's mother).
'such' going around 'ma' is 'SUMACH'.
(Other definitions for sumach that I've seen before include "This is a chum's tree, strangely" , "Shrub bright in autumn" , "As much (anag) - tree" .)