Plant lost time working with energy (9)
I believe the answer is:
mistletoe
'plant' is the definition.
The answer is an example.
'lost time working with energy' is the wordplay.
'working with' indicates an anagram (working with the letters).
'energy' becomes 'e' (physics symbol as in E=mc2).
'lost'+'time'='losttime'
'losttime' with letters rearranged gives 'mistleto'.
'mistleto'+'e'='MISTLETOE'
(Other definitions for mistletoe that I've seen before include "Evergreen Christmas decoration" , "Christmas kissing plant" , "Viscum album, much sought-after around Christmas" , "Something to stand under when kissing?" , "Yuletide hanging?" .)