Fruit tree, an energy source briefly retaining oxygen (5)
I believe the answer is:
carob
'fruit tree' is the definition.
(type of fruit tree)
'an energy source briefly retaining oxygen' is the wordplay.
'an energy source briefly' becomes 'carb' (short for carbohydrate; 'briefly' indicates a short form).
'retaining' indicates putting letters inside.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'carb' enclosing 'o' is 'CAROB'.
(Other definitions for carob that I've seen before include "St John's bread" , "that one could use for chocolate" , "Locust tree" , "(Seeds yielding) powder used as a chocolate substitute" , "Arabian tree with brownish-purple pods" .)