... under tree? Splendid! (7)
I believe the answer is:
sublime
'splendid' is the definition.
(thesaurus)
'under tree?' is the wordplay.
'under' becomes 'sub' (I've seen this before).
'tree?' becomes 'lime' (type of tree).
'sub'+'lime'='SUBLIME'
(Other definitions for sublime that I've seen before include "Unparalleled, extreme" , "Exalted, lofty" , "Transcendent" , "Magnificent" , "Majestic" .)