Present put on a tree, oddly (6)
I believe the answer is:
Here is my best explanation:
'present' is the definition.
(donating is a kind of presenting)
'put on a tree oddly' is the subsidiary indication.
'put on' becomes 'don' ('don' can be a synonym of 'put on'**).
'oddly' means one should take alternating letters.
The alternating letters of 'tree' are 'te'.
(Other definitions for donate that I've seen before include "Give money to charity", "Give as gift, say blood", "Present, give", "Contribute charitably", "Give (transplant organ)", "Give (eg to charity)", "Make a contribution", "Atoned (anag)", "Give - present" and "Give as a gift as to charity".)