Workshop I later rebuilt to conserve energy (7)
I believe the answer is:
atelier
'workshop' is the definition.
(workshop used by a designer or artist)
'i later rebuilt to conserve energy' is the wordplay.
'rebuilt' indicates an anagram.
'to conserve' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'i'+'later'='ilater'
'ilater' is an anagram of 'atlier'.
'atlier' going around 'e' is 'ATELIER'.
(Other definitions for atelier that I've seen before include "room for artist" , "I relate to workshop used by artist" , "Designer's workshop" , "Artist's workroom" , "Tie earl up in the artist's place" .)