Tree, one in forest shedding bark unusually (5)
I believe the answer is:
osier
'tree' is the definition.
(osier is a kind of tree)
'one in forest shedding bark unusually' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'shedding bark' indicates the central letters (the outside or bark or the word is removed).
'unusually' is an anagram indicator.
The middle of 'forest' is 'ores'.
'ores' is an anagram of 'oser'.
'i' going into 'oser' is 'OSIER'.
(Other definitions for osier that I've seen before include "Weaving cane for Rosie" , "Willow for basket making" , "Basket maker" , "Kind of willow used in basketwork" , "Wood" .)