Warned bad lad about damaged tree (7)
I believe the answer is:
alerted
'warned' is the definition.
(alerting is a kind of warning)
'bad lad about damaged tree' is the wordplay.
'bad lad' becomes 'ald' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'damaged' indicates an anagram.
'tree' is an anagram of 'erte'.
'ald' placed around 'erte' is 'ALERTED'.
Can you help me to learn more?
(Other definitions for alerted that I've seen before include "Related about warning notified" , "Altered, having been put on guard" , "Warned, made aware" , "Given signal" , "Tipped off" .)