Dwell on origin of oak tree (5)
I believe the answer is:
olive
'tree' is the definition.
(I've seen this before)
'dwell on origin of oak' is the wordplay.
'dwell' becomes 'live' ('live' can be a synonym of 'dwell').
'on' is a charade indicator (letters next to each other).
'origin of' indicates taking the first letters.
The initial letter of 'oak' is 'o'.
'live' after 'o' is 'OLIVE'.
(Other definitions for olive that I've seen before include "Yellow-green colour like Mediterranean fruit" , "Girl's name; tree" , "Cocktail fruit" , "This branch is offered to show peace" , "Oil producer" .)