It preserves energy and oxygen when getting into tight corset (8)
I believe the answer is:
creosote
'it preserves' is the definition.
(I've seen this before)
'energy and oxygen when getting into tight corset' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'and' is a charade indicator (letters next to each other).
'oxygen' becomes 'O' (atomic symbol for oxygen).
'when getting into' means one lot of letters goes inside another.
'tight' is an anagram indicator (tight can mean drunk).
'corset' with letters rearranged gives 'creost'.
'o' placed inside 'creost' is 'creosot'.
'e' put after 'creosot' is 'CREOSOTE'.
(Other definitions for creosote that I've seen before include "Wood-preserving oil" , "Fencing gear, perhaps" , "Oily liquid obtained from tar" , "Oily, smelly wood preservative" , "Wood preserver made from coal tar" .)