It's a different tree, lad! (7)
I believe the answer is:
altered
'it's a' is the definition.
I can't judge whether this defines the answer.
'different tree lad' is the wordplay.
'different' indicates an anagram (letters placed in a different order).
'tree'+'lad'='treelad'
'treelad' is an anagram of 'ALTERED'.
Can you help me to learn more?
(Other definitions for altered that I've seen before include "Change" , "Changed, adapted" , "Different" , "Reformed" , "Changed, modified" .)