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 justify this - if you can you should believe this answer much more).
'about' indicates putting letters inside.
'damaged' is an anagram indicator.
'tree' with letters rearranged gives 'erte'.
'ald' going 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" .)