Maintained donkey damaged tree, on the day (8)
I believe the answer is:
asserted
'maintained' is the definition.
('assert' can be a synonym of 'maintain')
'donkey damaged tree on the day' is the wordplay.
'donkey' becomes 'ass' (I've seen this before).
'damaged' is an anagram indicator.
'on' means one lot of letters goes inside another (as in clothing \'on\' a person).
'the day' becomes 'd'.
'tree' is an anagram of 'erte'.
'ass' going into 'erte' is 'asserte'.
'asserte'+'d'='ASSERTED'
(Other definitions for asserted that I've seen before include "Confidently declared to be so" , "Stated firmly, declared." , "Made firm statement about a dessert" , "Put" .)